High efficiency encoder and video information recording/reproducing apparatus

ABSTRACT

In a high-efficiency encoder which performs motion-compensation prediction, an intra-field is set every n fields. The presence of a scene change is detected. When a scene change occurs, a reference picture of motion-compensation prediction is switched, or the field immediately after the scene change is set as an intra-field.

This application is a divisional of application Ser. No. 08/599,488,filed on Nov. 15, 1995, now U.S. Pat. No. 5,841,474 which divisional ofapplication Ser. No. 08/011,243 filed on Jan. 29. 1993, now U.S. Pat.No. 5,479,264 the entire contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a digital signal recording/reproducingapparatus such as a video tape recorder (hereinafter, abbreviated as"VTR"), a video disk player and an audio tape recorder in which videoand audio signals are recorded and reproduced in the digital form, andparticularly to an apparatus which performs motion-compensationprediction on a video signal for compression-encoding.

2. Description of the Related Art

In a digital VTR for home use, data compression is indispensable in viewof cost and hardware size. Hereinafter, therefore, data compression willbe described taking mainly a digital VTR for home use as an example.

FIG. 1 is a schematic block diagram showing the structure of a digitalVTR for home use. The reference numeral 900 designates an input terminalthrough which an analog video signal such as a television signal isinput. The reference numeral 901 designates an A/D converter whichconverts the analog video signal into a digital video signal, 902designates a data compressor which compresses the digital video signalto reduce the information amount of the signal, 903 designates anerror-correction encoder which adds error-correcting codes to the codedsignal so that errors are corrected in the reproduction, 904 designatesa recording modulator which, in order to perform the recording,modulates the signal to codes suitable for the recording, 905 designatesa recording amplifier which amplifies the record signal, and 906designates a magnetic tape on which the record signal is recorded to bestored. The reference numeral 907 designates a head amplifier whichamplifies a signal reproduced from magnetic tape 906, 908 designates areproduction demodulator which demodulates the reproduced signal, 909designates an error-correction decoder which performs error-correctionon the reproduced and demodulated signal using the error-correctingcodes, 910 designates a data expander which reconstructs the compresseddata to its original form, 911 a D/A converter which converts thedigital video signal into an analog video signal, and 912 designates anoutput terminal.

Next, the data compressor (high-efficiency encoder) 902 will bedescribed. FIG. 2 is a block diagram of the high-efficiency encoderwhich employs one-way motion-compensation inter-frame prediction. Thereference numeral 1 designates an input terminal for a digital videosignal, 2 designates a blocking circuit which segments the input digitalvideo signal, 3 designates a subtracter which outputs as a differenceblock a difference signal between an input block and a prediction block,4 designates a difference power calculator which calculates the power ofthe difference block, 5 designates an original power calculator whichcalculates the AC power of the input block, 6 designates a determinerwhich compares the difference power with the original AC power todetermine whether the current mode is a prediction mode or an intramode, 7 designates a first switch which selectively outputs an encodedblock in accordance with the determined mode, 8 designates a DCT circuitwhich performs on the encoded block the discrete cosine transform(hereinafter, abbreviated as "DCT") that is an orthogonal transform, 9designates a quantizing circuit which quantizes a DCT coefficient,designates a first encoder which performs the coding suitable for atransmission path, and 11 designates the transmission path.

Reference numeral 12 designates an inverse quantizing circuit whichperforms inverse-quantization on the quantized DCT coefficient, 13designates an inverse DCT circuit which performs the inverse DCT on theinverse-quantized DCT coefficient, 14 designates an adder which adds aprediction block to the decoded block that is an output signal of theinverse DCT circuit 13 to generate an output block, 15 designates avideo memory which stores output blocks in order to performmotion-compensation prediction, 16 designates an MC circuit whichperforms motion estimation from a motion-compensation search blocksegmented from a past image stored in the video memory 15 and thecurrent input block, and performs motion-compensation prediction, 17designates a MIX circuit which combines a motion vector with a modesignal determined by the determiner 6, 18 designates a second encoderwhich codes the output of the MIX circuit 17, and 19 designates a secondswitch which switches the prediction blocks in accordance with the modedetermined by the determiner 6. The difference power calculator 4,original power calculator 5, determiner 6, inverse quantizing circuit12, inverse DCT circuit 13, adder 14, video memory 15, MC circuit 16 andsecond switch 19 constitute a local decoding loop 20.

Then, the operation will be described. Irrespective of an intra-field inwhich motion-compensation prediction is not performed or aprediction-field (inter-field) in which motion-compensation predictionis performed, input digital video signals are divided by the blockingcircuit 2 into input blocks in the unit of m pixels!×n lines! (where mand n are positive integers), and segmented. In order to obtain adifference block, the subtracter 3 calculates the difference in the unitof pixel between an input block and a prediction block. Then, the inputblock and the difference block are input into first switch 7. Thedifference power calculator 4 calculates the difference power of thedifference block. On the other hand, the original power calculator 5calculates the original AC power of the input block. The two calculatedpowers are compared with each other by the determiner 6 to control thefirst switch 7 so that the block having the smaller power is selected asthe encoding subject. More specifically, when the difference power issmaller than the original AC power, the determiner 6 outputs aprediction mode signal, and in contrast with this, when the original ACpower is smaller than the difference power, the determiner 6 outputs anintra mode signal.

The first switch 7 outputs the input block or the difference block as anencoded block in accordance with the mode signal determined by thedeterminer 6. When the processed image is in the intra-field, however,the first switch 7 operates so that all of the encoded blocks are outputas input blocks. FIG. 3 illustrates this switching operation. Theordinary mode is a mode where, in a step of motion-compensationprediction which is completed in four fields as shown in FIG. 4, firstfield F1 of the four fields is always an intra-field and the succeedingsecond, third and fourth fields F2, F3 and F4 are prediction-fields.

The encoded block selected by the first switch 7 is converted into DCTcoefficients by the DCT circuit 8, and then subjected to the weightingand threshold processes in the quantizing circuit 9 to be quantized topredetermined bit numbers respectively corresponding to thecoefficients. The quantized DCT coefficients are converted by the firstencoder 10 into codes suitable for the transmission path 11 and thenoutput to the transmission path 11.

The quantized DCT coefficients also enter into the local decoding loop20, and the image reproduction for next motion-compensation predictionis performed. The quantized DCT coefficients which have entered into thelocal decoding loop 20 are subjected to the inverse weighting andinverse quantizing processes in the inverse quantizing circuit 12. Then,the DCT coefficients are converted into a decoded block by inverse DCTcircuit 13. The adder 14 adds the decoded block to a prediction block inthe unit of pixel to reconstruct the image. This prediction block is thesame as that used in the subtracter 3. The output of the adder 14 iswritten as an output block in a predetermined address of the videomemory 15. The memory capacity of the video memory 15 depends on thetype of the employed predictive method. Assuming that the video memory15 consists of a plurality of field memories, the reconstructed outputblock is written in a predetermined address. A block which is segmentedfrom an image reconstructed from past output blocks and is in the motionestimation search range is output from the video memory 15 to the MCcircuit 16. The size of the block in the motion estimation search rangeis i pixels!×j lines! (where i≧m, j≧n, and i and j are positiveintegers). Data in the search range from the video memory 15 and aninput block from the blocking circuit 2 are input to the MC circuit 16as data, thereby extracting motion vectors. As a method of extractingmotion vectors, there are various methods such as the total search blockmatching method, and the tree search block matching method. Thesemethods are well known, and therefore their description is omitted.

The motion vectors extracted by the MC circuit 16 are input to the MIXcircuit 17, and combined therein with the mode signal determined by thedeterminer 6. The combined signals are converted by the second encoder18 into codes suitable for the transmission path 11, and then outputtogether with the corresponding encoded block to the transmission path11. The MC circuit 16 outputs as a prediction block signals which aresegmented from the search range in the size (m pixels!×n lines!), whichis equal to that of the input block. The prediction block to be outputfrom the MC circuit 16 is produced from past video information. Theprediction block is supplied to second switch 19, and output from therespective output terminal of the switch in accordance with the field ofthe currently processed image and the mode signal of the decoded block.Namely, the prediction block is output from one of the output terminalsof the second switch 19 to the subtracter 3 in accordance with theprocessed field, and from the other output terminal in accordance withthe mode signal of the current decoded block and the processed field.

As a predictive method used in such a circuit block, for example, themethod shown in FIG. 4 may be employed. In this method, an intra-fieldis inserted after every three fields, and the three intermediate fieldsare set as prediction-fields. In FIG. 4, first field F1 is anintra-field, and the second, third and fourth fields F2, F3 and F4 areprediction-fields. In the prediction by this method, second field F2 ispredicted from first field F1 which is an intra-field, third field F3 ispredicted in a similar manner from first field F1, and fourth field F4is predicted from reconstructed second field F2.

Initially, first field F1 is blocked in the field and subjected to theDCT. Then, first field F1 is subjected to the weighting and thresholdprocesses and quantized, and thereafter encoded. In the local decodingloop 20, the quantized signals of first field F1 are decoded orreconstructed. The reconstructed image is used in motion-compensationprediction for second and third fields F2 and F3. Then,motion-compensation prediction is performed on second field F2 usingfirst field F1. After the obtained difference block is subjected to theDCT, encoding is performed in a similar manner as in first field F1. Inthis case, when the AC power of the input block is smaller than thepower of the difference block, the input block in place of thedifference block is subjected to the DCT, and thereafter encoding isperformed in a similar manner as in first field F1. Second field F2 isdecoded and reconstructed in the local decoding loop 20 in accordancewith the mode signal of each block, and then used in motion-compensationprediction for fourth field F4. In a similar manner as in second fieldF2, using first field F1, motion-compensation prediction and encodingare performed on third field F3. Motion-compensation prediction isperformed on fourth field F4 using second field F2 reconstructed in thevideo memory 15, and then, fourth field F4 is encoded in a similarmanner as in third field F3. Also in third and fourth fields F3 and F4,when the AC power of the input block is smaller than the power of thedifference block, the input block in place of the difference block issubjected to the DCT, and thereafter encoding is performed in a similarmanner as in first field F1.

For example, the digital VTR for home use shown in FIG. 1 is expected toachieve the high image quality and high tone quality. In order torealize this, it is essential to improve data compression, i.e.,performance of high-efficiency encoder. Therefore, there arise followingproblems in the above-described conventional predictive method.

In such a predictive method, since motion-compensation prediction isperformed using the video data of the one preceding field or frame,there arises a first problem that the capacity of the field memory orframe memory is increased and the hardware is enlarged in size.

In the conventional predictive method, when a scene change once occursin the unit of frame, it is difficult during encoding of the image afterthe scene change to perform the compression according tomotion-compensation prediction from the reference picture which wasobtained before the scene change, thereby causing a second problem thatthe total amount of codes is increased. If inter-framemotion-compensation prediction is performed on the whole sequentially inthe temporal direction, it may be possible to suppress the increase inthe data amount to a minimum level even when a scene change occurs. Inthe case of encoding interlace images without scene change and with lessmotion, however, there is a tendency that the data amount is increasedas a whole. In a predictive method in which third and fourth fields F3and F4 are adaptively switched from first, second and third fields F1,F2 and F3 as shown in FIG. 5, there is a drawback that the capacity ofthe field memory or frame memory is increased and the hardware isenlarged in size. FIG. 6 shows the data amount and S/N ratio of aluminance signal, for example, when an image A with scene changes isprocessed by the predictive method of FIG. 4 or the predictive method ofFIG. 5. In the image A, a scene change occurs in the unit of frame. FIG.6 also shows the data amount and S/N ratio of a luminance signal in whenan image B without scene changes is processed by the predictive methodof FIG. 4 or the predictive method of FIG. 5. In this case, for theimage A with scene changes, the predictive method of FIG. 5 isadvantageous, and, for the image B without scene changes, the predictivemethod of FIG. 4 is advantageous.

In the case that the encoding is done by performing prediction as in theprior art (FIGS. 4 and 5), there is a third problem that, when a scenechange occurs in a step of a motion-compensation prediction process, thequality of the image immediately after the scene change is deteriorated.This problem is caused, owing to the scene change, bymotion-compensation prediction which unsatisfactorily performs timecorrelation thereby increasing the information amount being generated.The information amount generated in this way can compare with the levelof the information amount of a usual intra-field. For the generatedinformation amount, the field having this information amount is used asthe prediction-field, and therefore, the information amount iscompressed to the level of the information amount of theprediction-field, resulting in the image quality of the field after ascene change being substantially deteriorated. FIG. 7 shows a change ofthe information amount of images for five seconds when encoding isperformed by a conventional predictive method. In this case, the averagefor five seconds is less than 20 Mbps!, but a scene change exists at aposition A, thereby increasing the information amount. The change of theS/N ratio in this case is shown in FIG. 8. Although there is no greatdeterioration in the portion of the scene change, the decrease of theinformation amount makes the S/N ratio deteriorated. When that field isused in the next motion-compensation prediction, it is necessary toperform motion-compensation prediction on the image with thedeteriorated image quality and the reduced time correlation, the resultbeing that the information amount being generated is again increased.This vicious cycle continues until the next refresh field is processed.If deterioration of the image quality occurs in this way, even though itis immediately after a scene change, that means a digital videorecording/reproducing apparatus, which is required to have a high imagequality, fails to perform up to this level of quality. In a VTR for homeuse which is one of the digital video recording/reproducing apparatuses,for example, functions such as a trick play and edition areindispensable, and, when such a function is performed, remarkabledeterioration occurs in the image quality.

As conventional VTRs for home use of helical scanning type, there areVHS type, β type and 8-mm type VTRs. Here-inafter, a VTR of 8-mm typewill be described as an example of a prior art. FIG. 9 is a diagramshowing the tape format according to the 8-mm VTR standard, and FIG. 10is a diagram showing the format of one track. FIG. 11 is a diagramshowing the relationship between a rotary head drum and a magnetic tapewound around it, and FIG. 12 is a graph showing the frequency allocationof each signal according to the 8-mm VTR standard. In an 8-mm VTR forthe NTSC system or PAL system, a video signal is recorded by a colorundermethod which is a basic recording method for VTRs for home use. Theluminance signal is frequency-modulated with a carrier of 4.2 to 5.4MHz, chroma signal subcarrier is converted into a low frequency signalof 743 kHz, and the two signals are subjected to the frequency multiplexrecording. The recording format on a tape is as shown in FIG. 9. Allsignals required for a VTR at least including a video signal (luminancesignal, color signal), audio signals and tracking signals are subjectedto the frequency multiplex recording by the rotary video head. Thefrequency bands are shown in FIG. 12.

In FIG. 9, magnetic tracks 401 and 402 of a video signal track portion410 are tracks for a video signal, and each corresponds to one field.Magnetic tracks 403 and 404 indicated with oblique lines in an audiosignal track portion 411 are magnetic tracks for audio signals. A cuetrack 405 and audio track 406 for a fixed head are respectively set onthe both edges of the tape. Since the control track on the tape edge isnot used in an 8-mm VTR, this track can be used as the cue track forperforming specific point searching, addressing the contents ofrecording or the like. The width of one track (track pitch) is 20.5 μm,and is slightly greater than that in the economy recording mode of βtype and VHS type (19.5 μm in β-7, 19.2 μm in the 6-hour mode of VHS).No guard band for preventing a crosstalk from occurring is set betweentracks. Instead, azimuth recording using two heads is employed in orderto suppress a crosstalk.

Next, a specific example of the operation of a conventional apparatuswill be described with reference to FIGS. 13 to 16. FIG. 13 is a blockdiagram of a conventional VTR. A video signal given to a video signalinput terminal 201 is supplied to a video signal processing circuit 203and also to a synchronizing signal separating circuit 204. The outputsignal of the video signal processing circuit 203 is fed through gatecircuits 205 and 206 to adders 213 and 214. In contrast, a verticalsynchronizing signal which is an output of the synchronizing signalseparating circuit 204 is supplied to delay circuits 207 and 208. The Qoutput of the delay circuit 207 which combines with the synchronizingsignal separating circuit 204 to constitute head switch pulse generationmeans is supplied as a gate pulse to the first gate circuit 205 and alsoto a fourth gate circuit 212 which will be described later. The Q outputis supplied as a gate pulse to the second gate circuit 206 and also to athird gate circuit 211 which will be described later. The output signalof the delay circuit 208 is supplied to a time-base compressing circuit209 and also to an erasing current generator 240.

An audio signal given to an audio signal input terminal 202 is suppliedthrough the time-base compressing circuit 209, a modulating circuit 210and a switch 241 for switching between the recording and the erasing, tothe third and fourth gate circuits 211 and 212. The output of theerasing current generator 240 is supplied through the switch 241 to thethird and fourth gate circuits 211 and 212. The output signals of thethird and fourth gate circuits 211 and 212 are supplied to the adders213 and 214, respectively. The output signal of the adder 213 is givento a rotary transformer 217 through a changeover switch 215 forswitching between the recording and the erasing. The output signal ofthe rotary transformer 217 is given to a rotary magnetic head 221through a rotation shaft 219 and a rotary head bar 220, so that arecording current or an erasing current flows into a magnetic tape 223.

The output signal of the adder 214 is given to a rotary transformer 218through a switch 216 which is used for switching between recording andthe erasing and is interlocked with the switch 215. The output signal ofthe rotary transformer 218 is given to another rotary magnetic head 222through the rotation shaft 219 and the rotary head bar 220, so that arecording current or an erasing current flows into the magnetic tape223. The magnetic tape 223 is guided by guide posts 224 and 225 placedon the either sides of a table guide drum 226 which has rotary magneticheads 221 and 222 built in, and is run at a constant speed in thedirection of arrow 227, by a well known magnetic tape running device(not shown) which consists of capstans and pinch rollers. The tableguide drum 226 may have a well known structure, and therefore itsspecific description is omitted.

In the reproduction process, a signal reproduced by the rotary magnetichead 221 is supplied to a separating circuit 228 through the rotary headbar 220, the rotation shaft 219, the rotary transformer 217 and theswitch 216. On the other hand, a signal reproduced by the rotarymagnetic head 222 is supplied to a separating circuit 229 through therotary head bar 220, the rotation shaft 219, the rotary transformer 218and the switch 216. One of the outputs of the separating circuit 228 andone of the outputs of the separating circuit 229 are supplied to anadder 230. The other output of the separating circuit 228 and the otheroutput of the separating circuit 229 are supplied to an adder 231. Theoutput signal of the adder 230 is supplied to a video signal outputterminal 233 through a video signal processing circuit 232. In contrast,the output signal of the adder 231 is supplied to an audio signal outputterminal 237 through time-base correcting circuit 234, a demodulatingcircuit 235 and a time-base expanding circuit 236.

Then, the operation will be described. A video signal given to the videosignal input terminal 201 is converted into an FM signal by the videosignal processing circuit 203. When the video signal includes achrominance signal, the chrominance signal is converted into a lowfrequency signal of less than about 1.2 MHz. There will be no problemthat, for example, the phase of the chrominance signal is shifted by 90deg. or inverted every 1 H (horizontal scanning interval) as means foreliminating an adjacent color signal. This is a technique of eliminatinga crosstalk between tracks with using the line correlation ofchrominance signal. Such processed video signal is supplied to first andsecond gate circuits 205 and 206.

On the other hand, since the video signal is given also to thesynchronizing signal separating circuit 204, a vertical synchronizingsignal is obtained at the end of the output of the circuit. The verticalsynchronizing signal is supplied to the delay circuits 207 and 208. Thedelay circuit 207 has functions of dividing an input signal into a halffrequency and delaying a signal. From the ends of Q and Q outputs of thedelay circuit 207, pulse signals Q and Q for switching the heads andshown in FIGS. 14(b) and 14(c) are supplied to the first and second gatecircuits 205 and 206, respectively. In order to clarify the relationshipin phase between these pulse signals Q and Q and the input video signal,the waveform of the input video signal is shown in FIG. 14(a). From theends of outputs of first and second gate circuits 205 and 206, theprocessed video signals are output as shown in FIGS. 15(a) and 15(b)during the periods in which the pulse signals Q and Q are at H level.These signals are added to a modulated compressed audio signal anderasing signal which will be described later, by adders 213 and 214, andthen supplied to switches 215 and 216. The compressed audio signal issubjected to modulation suitable for the tape and head system(preferably, the pulse code modulation (PCM), or FM, PM, AM or the like,or in certain cases, the non-modulation AC bias recording), by themodulating circuit 210. Particularly, PCM is advantageous because a highS/N ratio can be expected and well known error correction means can beused for the drop-out, etc. The modulated compressed audio signal isgiven through the switch 241 to the third and fourth gate circuits 211and 212 to which the pulse signals Q and Q are respectively supplied.These gate circuits 211 and 212 output the compressed audio signal tothe adders 213 and 214 during the periods in which the pulse signals Qand Q are at H level.

The erasing current generator 240 generates an erasing current of acertain frequency (for example, 100 kHz). The timing of starting theoscillation of the erasing current is controlled by a trigger signal Twhich is obtained by delaying the vertical synchronizing signal in thedelay circuit 208. The erasing current is output through the switch 241to the third and fourth gate circuits 211 and 212 to which the pulsesignals Q and Q are respectively supplied, and supplied to the adders213 and 214. In the same manner as the recording of compressed audiosignals, during the periods in which Q pulse signals Q and Q are at Hlevel. FIGS. 16(a) and 16(b) shows the waveforms of the output currentsof the adders 213 and 214, i.e., time-multiplexed signals of a processedvideo signal A and a processed audio signal B or the erasing signal.These signals are supplied via the above-mentioned paths to the rotarymagnetic heads 221 and 222, whereby the magnetic pattern of a tape shownin FIG. 9 is obtained.

During the reproduction process, the moving contacts of the switches 215and 216 are positioned at fixed contacts P. This allows the two-channelreproduction signal reproduced by the rotary magnetic heads 221 and 222to be respectively transmitted through the rotary head bar 220, therotation shaft 219, the rotary transformers 217 or 218 and the switches215 or 216, and to be respectively separated into a video signal and anaudio signal on the time-base in separating circuits 228 and 229. Theseparated video signals are converted by the adder 230 into aone-channel video signal which is continuous in terms of time, and thensupplied to the video signal processing circuit 232. The video signalprocessing circuit 232 reconstructs the original video signal from theinput signal, and outputs the reconstructed signal to the video signaloutput terminal 233. On the other hand, the separated audio signals areconverted into a one-channel of signal by the adder 231, and thensupplied to the time-base correcting circuit 234. The time-basecorrecting circuit 234 consists of a semiconductor memory device such asa CCD (charge-coupled device) and a BBD (bucket brigade device), andeliminates time-base variations (so-called jitter and skew distortion)of the tape and head system. The output signal of the time-basecorrecting circuit 234 is demodulated to the original compressed audiosignal by the demodulating circuit 235. The demodulated signal is thenconverted into the original audio signal by the time-base expandingcircuit 236 consisting of a semiconductor memory device such as a CCDand a BBD, and output to the audio signal output terminal 237.

As described above, in an 8-mm VTR, video signals and audio signals forone field are recorded on and reproduced from one track on a tape.

FIG. 17 is a block diagram showing the configuration of a conventionalvideo information recording/reproducing apparatus. In FIG. 17, a digitalVTR of the D1 or D2 method which is used for business or broadcastinguse is shown. The reference numeral 101 designates an A/D converterwhich converts an analog video signal into a digital video signal, 102designates an error-correction encoder which adds error-correctingcodes, 103 designates a modulator which modulates the digital signal toa signals suitable for the recording on a magnetic tape, 104 designatesa rotary head drum, 105 designates a magnetic tape, 106 designatesmagnetic head for recording and reproduction, 107 designates ademodulator which demodulates the reproduced signal, 108 designates anerror-correction decoder which detects and corrects a transmissionerror, and 109 designates a D/A converter which converts the digitalvideo signal into an analog video signals.

FIG. 18 shows the tape formats of the two methods. In the both methods,a video signal and a 4-channel audio signal are recorded in differentpositions in the same track. In the D1 method, an audio signal isrecorded in the center of a track, and, in the D2 method, at the ends ofa track. When a video signal and an audio signal are recorded in thesame track, components such as a magnetic head and an amplifying circuitwhich are necessary for recording and reproducing can be used in commonfor a video signal and an audio signal, and furthermore, a parity coderequired for the error correction as described later and a circuit forgenerating the parity code can be used in common.

FIG. 19 shows the overall specifications of the D1 and D2 methods, FIG.20 shows the specifications of the tape formats, and FIG. 21 shows thespecifications of the tape running systems. The area recording densitywith guard bands being taken into account is 21.5 μm² /bit in the D1method, and 16.6 μm² /bit in the D2 method. In the D1 method, guardbands are set between recording tracks, but, in the D2 method, guardbands do not exist. As a result, the track density of the D2 method ishigher than that of the D1 method by about 15%, which contributes to thelong-time recording by the D2 method. On the other hand, when guardbands do not exist, it is more likely to reproduce a signal of anadjacent track in addition to a signal of the track originally intendedto be reproduced. In order to cope with this crosstalk between tracks inthe reproduction process, the D2 method employs the azimuth recordingsystem. Generally, a recording magnetic head and a reproduction magnetichead are so positioned that their head gaps form the equal angle with amagnetic track. If the two head gaps are arranged so as to form an anglewith each other, the level of a reproduced signal shows an attenuationcharacteristic. The azimuth angle θ in the D2 method is about ±15 deg.as shown in FIG. 20. As a result, even if a signal from an adjacenttrack is mixed in signals, to be reproduced the unnecessary component isattenuated. Accordingly, even if guard bands do not exist, the effect ofthe crosstalk is reduced. Since the loss due to the azimuth angle cannotbe expected for DC components, however, signals to be recorded arerequired to have no DC component. Therefore, the D2 method employs amodulation system which does not include DC components.

In a digital recording, it is not necessary to record a video signalduring the entire period. In a blanking interval, a video signal has aconstant waveform irrespective of the contents of an image. Since thiswaveform can be synthesized after the reproduction, in both the D1 andD2 methods, the recording is performed only during the effective videoperiod. Also a color burst signal appearing in a blanking interval of anNTSC signal can be synthesized after the reproduction. This is becausethe sampling phase in the D2 method is set to the I and Q axes and thephase of the color burst (lagging behind Q axis by (180+33) deg.) can bedetermined using a reproduced sampling clock.

FIG. 22 shows the ranges in which pixels can be actually recorded in theD1 and D2 methods. These effective pixels are divided into severalsegments. In the D1 method, pixels of 50 scanning lines constitute onesegment, and, in the D2 method, pixels of 85 scanning lines constituteone segment. In other words, pixels of one field constitute fivesegments in the D1 method, and three segments in the D2 method.

When a video signal in a segment is to be recorded, it is divided intofour channels in the D1 method, and into two channels in the D2 method.As a result, the number of pixels per one channel of one segment is{(720+360×2)/4}×50=360×50=18,000 in the D1 method, and(768/2)×85=384×85=32,640 in the D2 method. Channels are distributed sothat they are uniformly dispersed on a screen. Accordingly, even whenthe characteristics of a specific channel are deteriorated, code errorscaused by this deterioration are not concentrated on one portion of thescreen so as to be inconspicuous. Therefore, the effect of correction onerrors which have not been corrected is also enormous.

In both the D1 and D2 methods, two kinds of error-correction codes whichare respectively called an outer code and an inner code are usedtogether. In an actual process of generating outer and inner codes, anoperation of rearranging the sequence of the codes is performed. Thisoperation is called shuffling. The shuffling disperses the effect ofcode errors, improves the correction capability, and reduces the displaydeterioration caused by errors which have not been corrected. Theshuffling process consists of the shuffling for one scanning line whichis performed before the generation of an outer code, and the shufflingwhich is performed in one sector after the addition of an outer code andbefore the generation of an inner code. As described above, in a VTR ofthe D1 or D2 method, video signals and audio signals for one field arerecorded in a plurality of tracks on a tape.

In order to record all information of standard television signals ofcurrently used NTSC and PAL systems, in a VTR for home use, the carrierfrequency of an FM luminance signal is raised and the bandwidth anddeviation are increased so as to improve the resolution and C/N ratio.However, a VTR for home use still fails to match with a VTR for businessuse in S/N ratio, waveform reproducibility, etc. The down sizing of aVTR is highly expected to be achieved, and, there are demands forfurther improvement in performance as well as realization of VTR whichis light and compact. Hence, it is difficult to attain the desiredperformance by only improving the present techniques. On the other hand,in the field of VTRs for business use and broadcasting use, rapidadvance in digitalization of an apparatus has been made to achievemultifunction and high performance in the apparatuses, and most of VTRsfor broadcasting use are replaced with digital VTRs. However, a digitalVTR consumes a large amount of tape, which obstacles to achieveprolongation of the recording time and the down sizing.

Recently, in view of the redundancy of information contained in animage, studies on compressing recorded information have been activelyconducted, and the application of the results of these studies to a VTRis being examined. It is expected to realize a VTR which is compact andlight, has high image quality and can operate long-time by achievinghigh image quality, and reduction of tape consumption due to highdensity recording and information compression which are inherent in thedigital recording.

FIG. 23 shows a communication apparatus of a high-efficiency encodedvideo information compression method (according to CCITT H. 261, etc.)which is used in the field of communication including a video telephoneand a video conference. The reference numeral 101 designates an A/Dconverter which converts an analog video signal into a digital videosignal, 110 designates a high-efficiency code encoder whichcompression-encodes a video signal, 112 designates a buffer memory whichis used for delivering generated compressed codes at a constant speed,102 designates an error-correction encoder which adds error-correctingcodes, 103 designates a modulator which modulates the digital signal toa transmission signal suitable for the communication, 114 designates atransmission path, 107 designates a demodulator which demodulates areceived signal to a digital signal, 108 designates an error-correctiondecoder which detects and corrects a transmission error, 113 designatesa buffer memory which is used for supplying compressed codes that havebeen received at a constant speed, in accordance with the request fromthe next stage, 111 designates a high-efficiency code decoder whichexpands the compressed video signal to the original signal, and 109designates a D/A converter which converts the digital video signal intoan analog video signal.

The redundancy of an input video signal always varies, and therefore,the amount of codes which are compression-coded using this redundancyalso varies. However, the amount of information which can be transmittedthrough the transmission path 114 is limited. In order to exhibit thebest of the performance, the variation of the code amount is bufferedusing the buffer memory 113, and the information amount is controlled tobe within a predetermined range so that overflow or underflow of amemory does not occur. FIG. 24 shows the buffer operation performed atthe receiving side. Data which have been received at a constant rate arestored in the buffer memory, and, when the data amount reaches the levelB0, decoding of the codes starts. At the time when data of d1 have beenconsumed for the display of the first picture and the decoding for thesecond picture starts, the amount of the accumulated data is B1. In thesame manner, data accumulation and data consumption are alternatelyrepeated. The amount of consumed data varies depending on the displayedpicture, but the average amount of consumed data is equal to thereceiving rate. The operation of the receiving side has been described.The operation of the transmitting side is performed in the entirelyopposite way to that of the receiving side.

Since the communication apparatus is controlled as described above, therelationship between fields of an input video signal and transmittedcodes is not clearly defined. Unlike an application in the field ofcommunication, a VTR is required to perform functions peculiar to a VTRand including a special reproduction different from normal reproductionsuch as a still reproduction, slow reproduction and high-speedreproduction, an assemble edition, and in insert edition. Therefore, itis desirable to clearly define the relationship between fields andtracks. In order to produce a practical VTR, it is essential to select arecording format which can solve these problems.

As a method of compressing a moving picture such as a television signal,there is a method using an intra-field (or intra-frame) in which theencoding is completed within an individual field (or frame)independently of another field (or frame), and a prediction-field (orprediction frame) in which the predictive encoding is performed usinginformation of another field (or frame). Generally, the informationamount of the intra-field (or intra-frame) in which the predictionbetween fields (or frames) is not used is two or more times the codeamount of the prediction-field (or prediction frame) in which theencoding is performed using the prediction between planes. When recordareas of the same size (number of tracks) are allocated to theintra-field (or intra-frame) and the prediction-field (or predictionframe), there arises a fourth problem that in the intra-field (orintra-frame), the record area is not sufficient and in theprediction-field (or prediction frame) the record area has a uselessportion.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a high-efficiency encoderwhich does not perform motion-compensation on an intra-field (orintra-frame) every n fields (or n frames) and which performsmotion-compensation prediction with an intra-field (or intra-frame) inother fields (or frames), thereby solving the first problem and reducingthe hardware size.

It is another object of the invention to provide a high-efficiencyencoder in which a reference picture is previously set using a methodfixed somewhat for a normal picture having no scene change, and, when ascene change occurs in the step of the motion-compensation predictionprocess, the reference picture is switched so as to suppress the amountof generated information as much as possible, thereby solving the abovemention second problem. Further, the S/N ratio of a picture can bemaintained while suppressing the increase in the hardware size.

It is a further object of the invention to provide a high-efficiencyencoder in which motion-compensation prediction is performed while areference picture is previously set using a method fixed somewhat for anormal picture having no scene change, and, when a scene change occursin the step of the motion-compensation prediction process, the pictureimmediately after the scene change is encoded in a field (or frame) asan intra-field (or intra-frame), thereby the third problem mentionedabove is solved and the deterioration of a picture quality is suppressedafter the scene change as much as possible.

It is a still further object of the invention to provide a videoinformation recording/reproducing apparatus in which signal s of aplurality of fields (or frames) are collected into one recording unit tobe recorded in a predetermined number of tracks, thereby solving theabove mentioned fourth problem and coping with the special reproductionand edition required in a VTR.

In a high-efficiency encoder of the invention, only an intra-field (orintra-frame) is used as a reference picture for motion-compensationprediction, and therefore the information compression in which thedeterioration of the image quality is not conspicuous can be performedwith a reasonable hardware size.

In the other high-efficiency encoder of the invention,motion-compensation prediction is usually performed using a fixedreference picture, and, when a scene change occurs in the step of themotion-compensation prediction process, the reference picture isadaptively switched, thereby suppressing the increase in the informationamount. In this case, for example, blocks which are judged to be in anintra mode are counted in order to determine whether a scene changeoccurred, and, when blocks of an intra mode, the number of which isgreater than a preset threshold, are generated, the reference picturefor the next field (or frame) is switched. Therefore, the amount ofgenerated codes can be suppressed and the S/N ratio of a reproducedpicture can be maintained at a high level, by monitoring the encodingmode of a block, determining whether a scene change occurred, andswitching the reference picture for motion-compensation prediction.

When a scene change is detected, the picture immediately after the scenechange makes an intra picture and the information amount is compressed,an encoding error is caused only by the difference in this field (orframe), thereby reducing the effect on the motion-compensationprediction process of the next field (or frame). Namely, whenmotion-compensation prediction is performed before and after a scenechange, the amount of information generated in the field (or frame) isincreased, such that even when the information is compressed, the effectof an encoding error increase. Therefore, when the same amount ofinformation as that of the generated information is encoded as anintra-field (or intra-frame), higher subjective evaluation can beobtained. In the further high-efficiency encoder of the invention, whena scene change occurs in a motion-compensation prediction process unit,the field (or frame) immediately after the scene change is encoded as anintra-field (or intra-frame). Accordingly, even when the informationamount of the field (or frame) is compressed to the level of aprediction-field (or prediction frame), by performing encoding of thefield as an intra-field (or intra-frame), the deterioration of the imagequality of a picture can be suppressed more efficiently.

In the video information recording/reproducing apparatus of theinvention, input signals of n fields (or n frames) are collected intoone recording unit block, and recorded in tracks of a predeterminednumber which is calculated from the amount of information to be recordedand the recording capacity of one track. The compression encoding isperformed on blocks which are collected into the recording unit in sucha manner that at least one intra-field (or intra-frame) is included inthe unit. Input television signals of n fields (or n frames) aresubjected as one recording unit block to the compression encoding by ahigh-efficiency encoder. The compression-encoded television signals of nfields (or n frames) are divided to be recorded in recording areas of mtracks. The reproduced signals of m tracks are restored to televisionsignals of n fields (or n frames) by a high-efficiency decoder.

The above and further objects and features of the invention will morefully be apparent from the following detailed description withaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of a digital VTR;

FIG. 2 is a block diagram showing the configuration of amotion-compensation prediction processing apparatus;

FIG. 3 shows the block selection operation in the motion-compensationprediction processing apparatus;

FIG. 4 is a diagram showing the relationship between fields in aconventional motion-compensation prediction process;

FIG. 5 is a diagram showing the relationship between fields in amotion-compensation a prediction process;

FIG. 6 shows code amounts and SIN ratios in conventionalmotion-compensation a prediction;

FIG. 7 shows the change of the information amount for five seconds incase that a reference picture is not switched;

FIG. 8 shows the variation of the S/N ratio for five seconds in the casethat a reference picture is not switched;

FIG. 9 is a diagram showing the tape format according to the 8-mm VTRstandard;

FIG. 10 is a diagram showing the format of one track according to the8-mm VTR standard;

FIG. 11 is a diagram showing the relationship between a rotational headdrum and a magnetic tape wound around it used in an 8-mm VTR;

FIG. 12 is a graph showing the frequency allocation of each signal inthe 8-mm VTR standard;

FIGS. 13A and B is a block diagram showing the configuration of aconventional video information recording/reproducing apparatus;

FIGS. 14A, B, and C is a timing chart showing the relationship in phasebetween pulse signals for switching a head and input video signals inthe video information recording/reproducing apparatus of FIG. 13;

FIGS. 15A and B is a waveform chart showing video signals processed bygate circuits in the video information recording/reproducing apparatusof FIG. 13;

FIGS. 16A and B is a waveform chart showing time-base multiplexedsignals in the video information recording/reproducing apparatus of FIG.13;

FIG. 17 is a block diagram showing the configuration of anotherconventional video information recording/reproducing apparatus;

FIG. 18 is a diagram showing the tape formats of VTRs of the D1 and D2methods;

FIG. 19 shows the overall specifications of VTRs of the D1 and D2methods;

FIG. 20 shows the specifications of the tape formats of VTRs of the D1and D2 methods;

FIG. 21 shows the specifications of the tape running systems of VTRs ofthe D1 and D2 methods;

FIG. 22 is a diagram showing the ranges of pixels recorded by VTRs ofthe D1 and D2 methods;

FIG. 23 is a block diagram showing the configuration of a communicationapparatus of a high-efficiency encoded video information compressionmethod;

FIG. 24 illustrates the buffer operation of the high-efficiency codecommunication apparatus;

FIG. 25 is a block diagram showing the configuration of ahigh-efficiency encoder according to the invention;

FIG. 26 is a diagram showing the relationship between fields in amotion-compensation prediction process;

FIG. 27 is a diagram showing the configuration of anotherhigh-efficiency encoder according to the invention;

FIG. 28 is a diagram showing the relationship between fields in amotion-compensation prediction process;

FIG. 29 shows simulation results obtained in the case that there is ascene change;

FIG. 30 shows simulation results obtained in the case that there is noscene change;

FIG. 31 is a block diagram showing the configuration of a furtherhigh-efficiency encoder according to the invention;

FIG. 32 is a flowchart of the operation of the high-efficiency encoderof FIG. 31;

FIG. 33 is a flowchart of an intra-field process in FIG. 32;

FIG. 34 is a flowchart of a prediction-field process in FIG. 32;

FIG. 35 shows the change of the information amount for five seconds inthe case that a reference picture is switched;

FIG. 36 shows the variation of the SIN ratio for five seconds in casethat a reference picture is switched;

FIG. 37 is a flowchart of another prediction-field process in FIG. 32;

FIG. 38 is a flowchart of a further prediction-field process in FIG. 32;

FIG. 39 is a flowchart of a reference picture switching judging processin FIG. 38;

FIG. 40 is a block diagram showing the configuration of a still furtherhigh-efficiency encoder according to the invention;

FIG. 41 shows the operation of selecting blocks in the high-efficiencyencoder of FIG. 40;

FIG. 42 is a diagram showing a switching between a reference picture andan intra-field in the high-efficiency encoder of FIG. 40;

FIG. 43 is a flowchart of an operation of the high-efficiency encoder ofFIG. 40;

FIG. 44 is a flowchart of a prediction-field process in FIG. 43;

FIG. 45 is a diagram showing another switching between a referencepicture and an intra-field in the high-efficiency encoder of FIG. 40;

FIG. 46 is a flowchart of another operation of the high-efficiencyencoder of FIG. 40;

FIGS. 47A and B is a diagram showing the relationship between fields ina motion-compensation prediction process;

FIGS. 48A and B is a diagram showing the relationship between fields ina motion-compensation prediction process;

FIG. 49 is a diagram showing the switching of a reference picture;

FIG. 50 is a block diagram showing the configuration of a videoinformation recording/reproducing apparatus according to the invention;

FIGS. 51A and B is a diagram showing an example of the tape formataccording to the invention;

FIG. 52 is a block diagram showing the internal configuration of thehigh-efficiency encoder shown in FIG. 50;

FIG. 53 is a graph showing an example of the variation of the amount ofgenerated data for each frame; and

FIG. 54 is a diagram illustrating the relationship between recordedinformation in each field and the writing in tracks according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

Hereinafter, a first embodiment of the present invention will bedescribed with reference to the drawings. FIG. 25 is a block diagramillustrating the first embodiment of the invention. In FIG. 25, 1designates an input terminal for a digital video signal, 2 designates ablocking circuit, which forms into a block, a digital video signal inputthrough the digital video signal input terminal 1, 30 designates amotion-compensation predicting circuit which performsmotion-compensation prediction between the block output from theblocking circuit 2 and an intra-field and outputs a difference signalbetween an input block and a prediction block, 31 designates adeterminer which selects the one having the smaller sum of absolutevalues between the input signal from the blocking circuit 2 and aprediction difference signal from the motion-compensation predictingcircuit 30, 32 designates a first switch which selectively outputsencoded blocks output from the blocking circuit 2 and determiner 31,depending on the determined mode, 33 designates a orthogonal transformcircuit which performs the orthogonal transform on an encoded blockoutput from the first switch 32, 34 designates a quantizing circuitwhich quantizes the output of the orthogonal transform circuit 33, 11 isa transmission path, 35 designates a second switch which selects andoutputs the quantizing results output from the quantizing circuit 34,only in the case of an intra-field, 36 designates an inverse quantizingcircuit which inverse-quantizes the output of second switch 35, 37designates an inverse orthogonal transform circuit which performs aninverse orthogonal transform on the output of the inverse quantizingcircuit 36, and 38 designates a video memory which stores one field ofthe reproduced image of an intra-field output from the inverseorthogonal transform circuit 37 and outputs a reference picture in asearch range of the prediction field to the motion-compensationpredicting circuit 30.

As a predictive method used in such a circuit block, for example, themethod shown in FIG. 26 may be employed. In this method, an intra-fieldis inserted every four fields, and the three intermediate fields makesprediction-fields. In FIG. 26, first field F1 is an intra-field, andsecond, third and fourth fields F2, F3 and F4 are prediction-fields. Inthis method, second, third and fourth fields F2, F3 and F4 are predictedfrom first field F1. At first, first field F1, which is an intra-field,is blocked in the field, the orthogonal transform is performed, theblock is quantized, and then encoded. In the local decoding loop, thequantized signals of first field F1 are decoded or reconstructed. Thereconstructed image is used in motion-compensation prediction forsecond, third and fourth fields F2, F3 and F4. Then, motion-compensationprediction is performed on second field F2 using first field F1. Afterorthogonal transform is performed on the obtained difference blocks,encoding is performed in a similar manner as in first field F1. In thiscase, when the sum of the absolute values of the input blocks is smallerthan that of the difference blocks, orthogonal transform is performed onthe input blocks in place of the difference blocks, and thereafterencoding is performed in a similar manner as in first field F1. In asimilar manner as in second field F2, using first field F1, third andfourth fields F3 and F4 are also motion-compensation predictionprocessed and encoded. Also in third and fourth fields F3 and F4, whenthe sum of absolute values of the AC powers of the input blocks aresmaller than that of the difference blocks, orthogonal transform isperformed on the input blocks in place of the difference blocks.Thereafter encoding is performed in a similar manner as in first fieldF1.

Then, the operation will be described. Irrespective of the field (anintra-field or a prediction-field), digital video signals input throughthe digital video signal input terminal 1 are blocked by the blockingcircuit 2 in the unit of, for example, 8 pixels×8 lines. Themotion-compensation predicting circuit 30 performs motion-compensationprediction on input blocks which are output from the blocking circuit 2in the case of a prediction-field, while using as a reference picturethe reproduced video data of an intra-field stored in the video memory38. The motion-compensation predicting circuit 30 performs motionestimation while setting the search range of motion estimation at, forexample, 16 pixels×16 lines, to obtain a motion vector, and furtherobtains the difference signal between the reference picture and theinput image in accordance with the motion vector obtained in the motionestimation. The difference signal is output together with the motionvector to determiner 31. The determiner 31 obtains the sum of absolutevalues of components of each of the input blocks output from theblocking circuit 2 and the difference blocks output from themotion-compensation predicting circuit 30. When the input blocks areindicated by I(i, j) (i, j=1 to 8), the sum of their absolute values isindicated by Is, the difference blocks are indicated by P(i, j) (i, j=1to 8), and the sum of their absolute values is indicated by Ps, Is andPs can be expressed by the following expressions: ##EQU1##

When Ps<Is, it is determined that the information amount of thedifference blocks is smaller than that of the input signal blocks, andthen the difference blocks are output together with the motion vectorsto first switch 32. In contrast, when Ps≧Is, it is determined that theinformation amount of the input signal blocks is smaller than that ofthe difference signal blocks, and then the input blocks are outputtogether with a forced intra signal indicating that the block is theforced intra block, in place of the motion vectors, to the first switch32.

The first switch 32 selects the output of the blocking circuit 2 in thecase of an intra mode, and the output of the determiner 31 in the caseof a prediction mode, and supplies the selected output to the orthogonaltransform circuit 33. The orthogonal transform circuit 33 performs, forexample, the two-dimensional DCT on each of the 8×8 blocks which areinput thereinto. The quantizing circuit 34 performs variable lengthcoding and quantizes the orthogonal transform coefficients output fromthe orthogonal transform circuit 33. Furthermore, in the case of aprediction mode, quantizing circuit 34 also quantizes the motion vectorsor the forced intra signals in addition to the orthogonal transformcoefficients, and outputs them together to a transmission path 11. Onthe other hand, in order to produce reference data for themotion-compensation prediction, only in the case of the intra-field, thesecond switch 35 outputs the orthogonal transform coefficients whichhave been quantized by the quantizing circuit 34, to the inversequantizing circuit 36. The inverse quantizing circuit 36inverse-quantizes and performs variable length decoding of the data onwhich have been performed variable length coding by the quantizingcircuit 34. The decoded data are output to inverse orthogonal transformcircuit 37. The inverse orthogonal transform circuit 37 performs, forexample, the inverse two-dimensional DCT on the data to reconstruct theblocks of the intra-field. The blocks of the intra-field which have beenreconstructed by the inverse orthogonal transform circuit 37 are storedin the video memory 38. The video memory 38 stores the reconstructedpictures of the intra-field for one field, as the reference data in thecase of the motion-compensation prediction, and outputs the referencepictures within the motion vector detection range to motion-compensationpredicting circuit 30.

In the embodiment described above, a block for the orthogonal transformhas the size of 8 pixels×8 lines. This size is not necessarily 8pixels×8 lines, and the block may have the size of n pixels×m lines.Similarly; it is not necessary to set the search range for motionvectors to 16 pixels×16 lines, and the search range may be set to kpixels×s lines (k≧n, s≧m). In the above embodiment, the predictiveencoding is completed every four fields. However, is not necessary tocomplete the predictive encoding every four fields, instead thepredictive encoding may be completed every arbitrary number of fields.Furthermore, it is not necessary to complete the predictive encodingevery arbitrary number of fields, instead predictive encoding may becompleted every arbitrary number of frames. According to the aboveembodiment, in the determiner 31, the output having the smaller sum ofabsolute values is selected from the outputs of the blocking circuit 2and motion-compensation predicting circuit 30, to be output to the firstswitch 32. Alternatively, without performing the motion-compensationdetermination, only the output of the motion-compensation predictingcircuit 30 may be output to the first switch 32.

Embodiment 2

According to Embodiment 1 described above, in the determiner 31, theoutput having the smaller sum of absolute values is selected from theoutputs of the blocking circuit 2 and motion-compensation predictingcircuit 30, to be output to the first switch 32. In a field wheredeterminer 31 selects the forced intra mode more frequently than theprediction mode, it may be judged that a scene change has occurred inthe field, and assuming that the whole field is in the intra modeencoding may be performed. An embodiment constructed so as to performthis operation is Embodiment 2 described below.

FIG. 27 is a block diagram showing the configuration of the secondembodiment. In the figure, 40 designates a determiner which selects theone having the smaller sum of absolute values, from an input block fromthe blocking circuit 2 and a prediction difference block from themotion-compensation predicting circuit 30, and which judges a fieldwhere the input block from the blocking circuit 2 is selected morefrequently, to be an intra-field, 41 designates a first field memorywhich stores the input blocks output from the blocking circuit 2 as dataof an intra-field, 42 designates a second field memory which stores theblocks of a prediction-field output from the determiner 40, and 43designates a first switch which, in the intra mode and when thedeterminer 40 judges that the forced intra mode more frequently occursthan the prediction mode, selects the output of the first field memory41 and outputs it to the orthogonal transform circuit 33, and which, inother case than the above-mentioned two cases, selects the output of thesecond field memory 42. FIG. 27

Further operation of the second embodiment shown in FIG. 27 will bedescribed. In the process between the digital video signal inputterminal 1 and motion-compensation predicting circuit 30, operation iscarried out in the same manner as those in the first embodiment, andtherefore the description is omitted. The determiner 40 selects the onehaving the smaller sum of absolute values of components, from inputblocks output from the blocking circuit 2 and difference blocks outputfrom the motion-compensation predicting circuit 30, and outputs it. Whenthe output from the motion-compensation predicting circuit 30 isselected, the determiner 40 outputs motion vectors and blocks ofdifference signal. When the output from the blocking circuit 2 isselected, motion vectors are output together with a signal indicative ofa forced intra-block. When the number of forced intra-blocks in onefield reaches or exceeds a value n, determiner 40 judges that a scenechange has occurred, and outputs a control signal so that the wholecurrent field is to be encoded by the intra mode.

The output of the determiner 40 is stored as data of the prediction modein the second field memory 42, and, after data for one field have beenstored, it is output to the first switch 43. In contrast, the output ofthe blocking circuit 2 is stored as data of the intra mode in the firstfield memory 41, and, after data for one field have been stored, it isoutput to the first switch 43. In the intra mode, and when thedeterminer 40 determines to force the intra mode on the field, the firstswitch 43 selects the output of the first field memory 41, and, inanother case, selects the output of second field memory 42. The outputof the first switch 43 is supplied to the orthogonal transform circuit33. In the succeeding process from the orthogonal transform circuit 33to the video memory 38, the operation is carried out in the same manneras those in the first embodiment, and therefore the description isomitted. However, when it is determined by the determiner 40 that ascene change occurs, it is required to update also the contents of thevideo memory 38. In this case, therefore, the second switch 35 operates,in the same manner as in the intra mode, to supply the output of thequantizing circuit 34 to the inverse quantizing circuit 36.

Hereinafter, examples in which encoding and decoding are respectivelyperformed by three kinds of predictive encoding methods shown in FIGS.26, 4 and 28 will be described. In the predictive encoding method shownin FIG. 28, prediction between fields is performed in a field, and thirdfield F3 is predicted from first field F1. Referring to the encodingmethods shown in FIGS. 26, 4 and 28 respectively as methods 1, 2 and 3,FIG. 29 shows results of simulation processes which were conducted onsample images for five seconds wherein a scene change exists. Similarly,FIG. 30 shows results of simulation processes which were conducted onsample images for five seconds wherein no scene change exists. For thesesample images, 4:2:2 component signals (Y: 720×240, Cb and Cr: 360×240,60 Fields/sec.) are used. As seen from the results shown in FIGS. 29 and30, with images having a scene change, method 3 is advantageous in theview point of the S/N ratio, but, for images having no scene change,there is little difference between methods 1 to 3. As a result, when ascene change occurs, the intra-field is employed forcedly, and ahigh-efficiency encoder of which hardware size is smaller than that of aconventional predictive encoder can be realized.

In Embodiment 2 described above, the intra mode is created every nfields and the succeeding (n-1) fields are prediction-encoded from anintra-field, and, when a scene change occurs, the intra-field isforcedly created and the remaining fields are predicted from the forcedintra mode. It is not necessary that an intra-field exists every nfields. Alternatively, when a forced intra-field is created, thesucceeding (n-1) fields which comes after the forced intra-field may beprediction-encoded. In the above-described embodiment, the predictiveencoding is performed in the unit of field. It is not necessary toperform predictive encoding in the unit of field, and predictiveencoding may be performed in the unit of frame.

As described above, since the high-efficiency encoders of Embodiments 1and 2 create an intra-field every n fields and in the other fields,perform motion-compensation prediction using this intra-field as areference picture, the hardware size including a calculation circuit forobtaining motion vectors can be reduced.

Embodiment 3

FIG. 31 is a block diagram showing the configuration of ahigh-efficiency encoder in Embodiment 3. In FIG. 31, 1-14 and 16-20designate the elements identical to those of the apparatus in FIG. 2.The reference numeral 50 designates a mode counter which counts thenumber of blocks of the intra mode, 51 designates a direction switchingcircuit which compares a predetermined number of blocks with the numberof blocks of the intra mode output from the mode counter and whichdetermines the reference picture for the next field, and 52 designates avideo memory which stores output blocks in order to performmotion-compensation prediction and which outputs the reference picturefor the next field as the search range.

Then, the operation will be described. Irrespective of an intra-field ora prediction-field, input digital video signals are segmented by theblocking circuit 2 into input blocks, a unit of which consists of mpixels!×n lines!. In order to obtain a difference block., subtracter 3calculates the difference in the unit of pixel between an input blockand a prediction block. In this way, an input block and a differenceblock are supplied to first switch 7. In order to calculate the power,the difference block is supplied also to difference power calculator 4and the difference power is calculated. In order to calculate the ACpower, the input block is supplied to the original power calculator 5and the original power is calculated. The outputs of the differencepower calculator 4 and original power calculator 5 are supplied to thediscriminating circuit 6 which in turn selects the one having thesmaller power from the two powers outputting the result to the firstswitch 7 as the mode signal. More specifically, when the differencepower is smaller than the original AC power, the prediction mode signalis output so that the first switch 7 is set to the prediction mode tooutput the difference block as an encoded block. When the original poweris smaller than the difference power, the intra mode signal is output sothat first switch 7 is set to the intra mode to output the input blockas an encoded block.

The mode signal from discriminating circuit 6 is input to the modecounter 50. Since the input mode signal is generated for every block ofa prediction-field, the mode counter 50 counts the number of blocksselecting the intra mode or prediction mode, from the blocks of onefield, and outputs the number of blocks selecting an intra mode orprediction mode to the direction switching circuit 51. The directionswitching circuit 51 compares a predetermined number of blocks (which issmaller than the total number of blocks of one field) with the number ofblocks selecting an intra mode, which number has been input from modecounter 50, and outputs a reference picture switching signal to videomemory 52. When the predetermined number of blocks is greater (orsmaller) than the number of blocks selecting an intra mode (or aprediction mode), direction switching circuit 51 outputs to video memory52 a reference picture switching signal so that the reference picture isnot to be switched. When the number of predetermined blocks is smaller(or greater) than the number of blocks selecting an intra mode (or aprediction mode), the direction switching circuit 51 outputs to thevideo memory 52 a reference picture switching signal so that thereference picture is to be switched.

The first switch 7 outputs an input block or a difference block as anencoded block, depending on the mode signal determined by thediscriminating circuit 6. In this case, when the input block is theintra-field, the first switch 7 surely outputs the input blocks asencoded blocks. The encoded blocks enter the DCT circuit 8 to beconverted into DCT coefficients. The DCT coefficients are subjected tothe weighting and threshold processes in the quantizing circuit 9 to bequantized to predetermined bit numbers each corresponding to thecoefficients. The DCT coefficients which have been quantized to therespective predetermined bit numbers are converted by the first encoder10 into codes suitable for transmission path 11 and then output totransmission path 11.

The DCT coefficients on which weighting, and threshold processes andquantization have been performed by the quantizing circuit 9 also enterthe local decoding loop 20, and are subjected to the inverse weightingand inverse quantizing processes in the inverse quantizing circuit 12.Then, the DCT coefficients on which inverse weighting and inversequantizing processes have been performed in the local decoding loop 20are converted into a decoded block by inverse DCT circuit 13. The adder14 adds the decoded block to a prediction block in the unit of pixel.This prediction block is the same as that used in the subtracter 3. Theresult of addition by the adder 14 is written as an output block in apredetermined address of the video memory 52. Video memory 52 switchesthe reference picture in response to the reference picture switchingsignal from the direction switching circuit 51, and outputs the motionestimation search range to the MC circuit 16. The size of the block inthe motion estimation search range is, for example, i pixels!×j lines!(where i≧m, j≧n). A block in the motion estimation search range outputfrom the video memory 52, and an input block from the blocking circuit 2are input into the MC circuit 16. The MC circuit 16 performs the motionestimation on each block to extract motion vectors of the input block.

The motion vectors extracted by the motion estimation in the MC circuit16 are input into the MIX circuit 17. The MIX circuit 17 combines themotion vectors from the MC circuit 16 with the mode signal determined bythe discriminating circuit 6. The motion vectors and mode signal whichhave been combined with each other in the MIX circuit 17 are convertedby the second encoder 18 into codes suitable for the transmission path11, and then output together with the corresponding encoded blocks tothe transmission path 11. The prediction block is output from the MCcircuit 16 in the form of a block which is blocked into the size (mpixels!×n lines!) equal to that of the input block from the motionestimation range. This prediction block is supplied to the second switch19, and output from the respective output terminal of the switch, inaccordance with the field of the input block the currently beingprocessed and the mode signal of the decoded block. Namely, theprediction block is output from one of the output terminals of thesecond switch 19 to the subtracter 3 in accordance with the processedfield, and from the other output terminal in accordance with the modesignal of the current decoded block and the processed field.

According to the present invention, in the case that a scene changeoccurs in the unit of frame when the predictive method shown in FIG. 4is used for an ordinary picture, the number of blocks, which select anintra mode in encoding the picture immediately after the scene change,increases, and the reference picture used thereafter can be switched asshown in FIG. 28.

The operation in Embodiment 3 is summarized below with reference toflowcharts in FIGS. 32, 33 and 34. FIG. 32 is a flowchart showing thewhole operation in Embodiment 3, FIG. 33 is a flowchart showing anintra-field process of step S103 in FIG. 32, and FIG. 34 is a flowchartshowing a prediction-field process of step S104 in FIG. 32.

Firstly, the field number fn indicative of the field in themotion-compensation prediction process unit is set to be 0 (step S101).Referring to FIG. 4, this field number fn will be described. Intra-fieldF1 which comes first in the motion-compensation prediction process unitis identified by the field number fn=0, intra-field F2 is identified bythe field number fn=1, next intra-field F3 is identified by the fieldnumber fn=2, and intra-field F4 which comes last in themotion-compensation prediction process unit is identified by fieldnumber fn=3. Since a motion-compensation process has just been started,the field to be initially processed is surely the first field in themotion-compensation prediction process unit and an intra-field, andtherefore, the field number fn is set to be 0 (fn=0) in step S101. Areference picture switching flag Rfn which functions as a flag fordetermining whether a scene change exists is set in a laterprediction-field process, but in this step the flag Rfn is set to be 0(Rfn=0) for initialization.

Then, the field number fn is checked to determine whether it is 0 orwhether the field is the first field in the motion-compensationprediction process unit and an intra-field (step S102). If the fieldnumber fn is 0 (fn=0), this field is processed as an intra-field (stepS103). In contrast, if the field number fn is not 0 (fn≠0), this fieldis processed as a prediction-field. These processes will be described indetail later. After each field is processed, the field number fn isincremented so as to indicate the next field (step S105). In an actualhardware, such a field number can be controlled by a microcomputersignal or the like.

It is judged whether the field number fn indicating the next field is anumber indicating a field within the motion-compensation predictionprocess unit (step S106). If the field number fn is a number notindicating a field within the motion-compensation prediction processunit, for example, as in FIG. 4, wherein since the motion-compensationprocess unit has been completed within four fields and the field numberfn of an intra-field has been set to be 0, if fn=4, it means that aseries of motion-compensation prediction units have been completed. Iffn<4, it is judged that the next field is still within themotion-compensation process unit, and the process is repeated.

When a series of the motion-compensation prediction process units havebeen completed, it is judged whether all of the required fields havebeen processed (step S107). This judgment can be done by, for example,checking the operation of an end switch of the high-efficiency encoder.If the next field is to be processed, in order to encode the nextmotion-compensation prediction process unit, the variables areinitialized, and the process is repeated. If the operation of thehigh-efficiency encoder has been completed, the encoding ends.

The intra-field process will be described with reference to theflowchart in FIG. 33. The field which is determined in step S102 in FIG.32 to be processed as an intra-field is segmented into the predeterminedsize of m pixels!×n lines! in the processed field (step S201). Then, anorthogonal transform such as DCT is performed on the blocks of that size(step S202). T he data on which orthogonal transform has been performedare quantized into a predetermined bit number which is set for eachsequence (step S203). In an orthogonal transform such as DCT, generally,a quantization is performed in such a manner that a larger bit number isassigned to a DC and low-order segments of AC, and a smaller bit numberis assigned to a high-order sequence of AC. The quantized data areconverted into codes suitable for transmission (step S204), and theencoded data are transmitted (step S205). It is judged by, for example,counting the number of processed blocks, whether the process of onefield has been completed (step S206). If the process of one field hasnot yet been completed, the process for the next block is pursued. Ifthe process of one field has been completed, the intra-field process isended.

The prediction-field process will be described with reference to theflowchart in FIG. 34. The field which is determined in step S102 in FIG.32 to be processed as a prediction-field is checked to judge whetherRfn-1=0, in the reference picture switch flag, in the process of theprevious field or whether a scene change has been detected in theprocess of the field preceding the field currently being processed (stepS301). If Rfn-1=0, motion-compensation prediction is performed using thereference picture in the same position as before (step S302). IfRfn-1=1, it means that a scene change has been detected when fieldnumber fn-1 being processed. Therefore, in the motion-compensationprediction for field number fn the reference picture is switched to apicture of a field in a position different from the previous position,and motion-compensation prediction is performed using the new referencepicture (step S303).

Then, a variable COUNT for counting the number of blocks which select anintra mode in one field to be processed is set to be 0 (step S304). Thevariable COUNT will be described in detail later. An input picture issegmented into the predetermined size of m pixels!×n lines! in theprocessed field (step S305). The blocks segmented into the size of m×nare subjected to motion-compensation prediction (step S306). Using thereference picture set in step S302 or S303, the difference in the unitof pixel between a predetermined area of a past image and a newlydivided block is input as a difference block into difference powercalculator 4, and a difference power P1 is calculated (step S307).Namely, by using such preset reference picture, the information amountgenerated by motion-compensation prediction can be reduced. Then, thenewly segmented block is input into original power calculator 5 tocalculate an original AC power P2 (step S308).

The calculated powers P1 and P2 are compared in magnitude with eachother (step S309). When the difference power P1 is smaller than theoriginal AC power P2, the difference block (the difference value of theblock subjected to motion-compensation prediction) is selected (stepS310). When the difference power P1 is greater than the original ACpower P2, the input block (the original which is still in block form isselected (step S311), and the number of times when an input block isselected as an encoded block or the number of blocks in one field whichare to be processed as an intra mode is counted (step S312). A variablethat functions as a counter in this step is the COUNT which has been setto be 0 in step S304. This counter is surely set to be 0 when theprocess in the unit of field starts, and counts the number of blockswhich select an intra mode in the processing of one field.

Each selected block is subjected to the orthogonal transform (stepS313), and quantized to a predetermined bit number which is set for eachsequence (step S314). In an orthogonal transform such as DCT, aquantization is performed in such a manner that a larger bit number isassigned to a DC and low-order sequences of AC, and a smaller bit numberis assigned to a high-order sequence of AC. The quantized data areconverted into codes suitable for transmission (step S315), and theencoded data are transmitted (step S316). For example, the number ofprocessed blocks is counted to judge whether the process of one fieldhas been completed (step S317). If the process of one field has not yetbeen completed, the process for the next block is pursued.

If the process of one field has been completed, the number of inputblocks which have been processed as encoded blocks in the process ofthat one field or the number of blocks which have selected an intra modeis compared with a preset threshold TH (step S318). The threshold TH isa predetermined number which is less than the number of blocks in onefield. When the total number of blocks in one field is 2,700, forexample, the threshold TH is set to be 1,000 which is less than 2,700.If the variable COUNT indicating the number times when input blocksbeing selected as encoded blocks is smaller than the threshold TH, thereis no scene change between the field (field number fn) which has beenjust processed and the reference picture which has been used inmotion-compensation prediction of that field, and the reference pictureswitch flag Rfn is set to be 0 (Rfn=0) so that the reference picture ina normal position is used as the reference picture for themotion-compensation of the next field (field number fn+1) (step S319).If the variable COUNT indicating the number of times when input blocksbeing selected as encoded blocks is greater than the threshold TH, thereis a scene change between the field (field number fn) which has beenjust processed and the reference picture which has been used inmotion-compensation prediction of that field. The reference picture forthe motion-compensation of the next field (field number fn+1) is thenswitched from the reference picture in a normal position to a field in aposition different from the position taken till then, for example, thefield which has just been processed and is positioned in a place where areference picture has not existed till then. For that purpose, thereference picture switch flag Rfn is set to be 1 (Rfn=1) (step S320). Inthis way, the reference picture switch flag Rfn is set and theprediction-field process ends.

FIG. 35 shows the change of the information amount for five seconds incase that the predictive encoding is performed according to theEmbodiment 3, and FIG. 36 shows the variation of the S/N ratio for fiveseconds in this case. Although a scene change exists at point B, theincrease of information amount is suppressed as compared with point Ashown in FIG. 7. Furthermore, there is no conspicuous deterioration ofthe S/N ratio.

Embodiments 4 and 5

In Embodiment 3, in order to select an encoded block from a differenceblock and an input block, their powers are calculated and compared witheach other, and the number of blocks selecting an intra mode arecounted.

According to Embodiment 4, in order to select an encoded block from adifference block and an input block, the entropy in each block iscalculated, and the entropy of the difference block is compared withthat of the input block in the same manner as in Embodiment 3 by thediscriminating circuit 6, to determine which block is to be selected asan encoded block.

According to Embodiment 5, in order to select an encoded block from adifference block and an input block, adding of absolute values of pixelsis performed in each block, the rth power of the sum of absolute valuesof the input block and that of difference block are calculated, and therth power of the sum of absolute values of the difference block iscompared with that of the input block in the same manner as inEmbodiment 3 by the discriminating circuit 6, to determine which blockis to be selected as an encoded block.

Embodiment 6

In Embodiment 3, the discriminating circuit 6 compares the power of aninput block with that of a difference block. According to Embodiment 6,when the power of an input block is to be compared with that of adifference block, at least one of the powers of the input and differenceblocks is provided with an offset, and then the two powers are comparedwith each other. For example, the power of the input block is providedwith a positive offset, and then compared with the power of thedifference block. When there is not a great difference in power betweenthe input and difference blocks, this configuration allows the number ofblocks selecting the difference power to be increased, therebypreventing an intra mode from being excessively created.

FIG. 37 is a flowchart of a prediction-field process in Embodiment 6. InFIG. 37, portions designated by the same step numbers used in FIG. 34are identical with those of FIG. 34. The processes from step S301 tostep S308 are the same as in Embodiment 3. A difference power P1calculated from a difference block is compared with a value which isobtained by adding a predetermined offset a to an original AC power P2calculated from an input block (original block) (step S330). This makesit difficult to obtain P1<P2+α compared with Embodiment 3, so that thenumber of blocks selecting an intra mode is reduced. As a result, anintra mode is prevented from being excessively created, and thegenerated information amount can be stably kept at a constant level. Thesucceeding processes from step S310 to step S320 are the same as inEmbodiment 3.

Embodiment 7 and 8

According to Embodiment 7, when the entropy of a difference block iscompared with that of an input block in a similar manner as inEmbodiment 4, at least one of the entropy of the input and the entropyof the difference block is provided with an offset, and then the twovalues are compared with each other. For example, the entropy of theinput block is provided with a positive offset, and then compared withthe entropy of the difference block. When there is not a greatdifference between the entropy of the input block and the entropy ofdifference block, this configuration allows the number of blocksselecting difference power to be increased, thereby preventing an intramode from being excessively created.

According to Embodiment 8, when the sum of absolute values of adifference block is compared with that of an input block in a similarmanner as in Embodiment 5, at least one of the rth power of the sum ofabsolute values of the input block and that of the difference block isprovided with an offset, and then the two values are compared with eachother. For example, the rth power of the sum of absolute values of theinput block is provided with a positive offset, and then compared withthe rth power of the sum of absolute values of the difference block.When there is not a difference greater than the offset between the rthpower of the sum of absolute values of the input and that of thedifference block, this configuration allows the number of blocksselecting the difference power to be increased, thereby preventing anintra mode from being excessively created.

Embodiment 9

In Embodiment 3, the mode counter 50 counts the number of all blocksselecting an intra mode among blocks for one field. In Embodiment 9,blocks for one field are not counted, but, at the time a mode signal ina predetermined number of blocks during one field is determined, theratio of the number of blocks selecting an intra mode to the totalnumber of blocks or the number of the blocks in which a mode signal hasbeen determined is supplied to the direction switching circuit 51. Basedon this ratio, a reference picture switching signal is output from thedirection switching circuit 51. This configuration allows the referencepicture for the next field to be determined even when encoding of allblocks for one field has not yet completed.

FIG. 38 is a flowchart of a prediction-field process in Embodiment 9. InFIG. 38, portions designated by the same step numbers used in FIG. 34are identical with those of FIG. 34. The processes from step S301 tostep S303 are the same as in Embodiment 3. After the reference picturefor motion-compensation prediction for the next field is set (steps S302and S303), the variable COUNT for counting the times when intra modesbeing generated in one field during the processing of the field or thenumber of blocks selecting input blocks as encoded blocks, and variableB for counting the number of blocks which have been processed in theprocessing of the one field till then are set to be 0 (step S340). Thesucceeding steps S305 to S316 are the same as those of in Embodiment 3.After encoding, the number of block which have been processed till thenis counted by incrementing the variable B one by one (step S341). Thevariable B changes from 0 to the maximum number of blocks which canexist in one field. The reference picture switching determinationprocessing for determining whether the reference picture formotion-compensation prediction of the next field is to be switched isperformed (step S342). The next step S317 is the same as that ofEmbodiment 3.

FIG. 39 is a flowchart of the reference picture switch determinationprocess in step S342 in FIG. 38. The process will be described withreference to FIG. 39. It is judged whether the reference picture switchflag Rfn is 0 (step S351). If the flag Rfn is not 0, the process isended. If the flag Rfn is 0, the ratio of the COUNT for counting thetimes when input blocks being as encoded blocks to the variable B forcounting blocks which have been processed in the processing of the onefield till then is compared with the threshold TH (step S352). If theratio is smaller than the threshold TH, the process is ended. If theratio is greater than the threshold TH, the flag Rfn is set to be 1(step S353), and the process is ended.

Embodiment 10

Embodiment 10 will be described with reference to FIG. 40 which showsthe configuration of the embodiment. In FIG. 40, the reference numerals1, 3 to 6, 8 to 16, 18 and 20 are the same as those used in theapparatus in FIG. 2. The reference numeral 60 designates a video memoryin which input pictures are stored, 61 designates an SC detectioncircuit which detects a scene change in a picture and outputs a signalindicative of this, 62 designates a first switch which switches from aninput block segmented from an original picture to a difference blockgenerated from a prediction block due to motion-compensation prediction,63 designates a MIX circuit in which a motion vector, the mode signal ofa block from the discriminating circuit 6 and the scene change (SC)detection signal from the SC detection circuit 61 are combined, and 64designates a second switch which switches a prediction block.

Then, the operation will be described. It is assumed thatmotion-compensation prediction is performed, for example, as shown inFIG. 4 and is completed within four fields. Digital video signals inputthrough the input terminal 1 are stored in the video memory 60. Thevideo memory 60 has a memory for at least two fields, and, while storingvideo signals of one of the two fields, blocks video data for scenechange detection or processing into a predetermined size are output fromthe other field. Namely, the video memory 60 firstly sends digital videosignals to SC detection circuit 61, and characteristics of a picture,for example, obtained from preset parameters, and the presence of ascene change is detected. Then, digital video signals are output fromone of the outputs of the video memory 60 while being blocked into thesize of, for example, m pixels!×n lines! (where m and n are positiveintegers). The size of m pixels!×n lines! corresponds to the block sizefor performing the two-dimensional orthogonal transform, and also to theblock size of a prediction block based on the motion-compensationprediction.

An input block which is obtained only by blocking an original outputfrom the video memory 60, and a difference block that is a differencebetween the input block and a prediction block which has been subjectedto motion-compensation prediction by the subtracter 3 are input to thefirst switch 62. The input block and the difference block arerespectively input to the original power calculator 5 and the differencepower calculator 4 in order to obtain the power of each block. Theoriginal power calculator 5 calculates the AC power of the input block,and the difference power calculator 4 calculates the power of thedifference block. The calculated AC power of the input block and thecalculated power of the difference block are input to the discriminatingcircuit 6. When the power of the difference block is smaller than thatof the input block, the discriminating circuit 6 outputs a predictionmode signal, and, when the power of the input block is smaller than thatof the difference block, the discriminating circuit 6 outputs an intramode signal. These signals are supplied as a mode signals to the firstswitch 62, the MIX circuit 63 and the second switch 64.

The first switch 62 to which the input and difference blocks are inputoutputs either of the blocks as an encoded block. For that purpose, thefirst switch 62 receives the scene change detection signal from the SCdetection circuit 61 and also the mode signal from the discriminatingcircuit 6, to determine the switch mode, and outputs either of the inputand difference blocks as an encoded block. The switching states at thistime are shown in FIG. 41. Since the process step of motion-compensationprediction completes within four fields as shown in FIG. 4, in theordinary mode an intra-field is the first field, a prediction-field thencontinues from the second field to the fourth field, an intra-field isagain the first field, and the above is repeated continuously. Withrespect to the detection of the presence and absence of a scene changeshown in FIG. 41, when the scene change detection signal from the SCdetection circuit 61 indicates the detection of a scene change, a signalof the presence is output, and, when the scene change detection signaldoes not indicate the detection of a scene change, a signal of theabsence is output. The discriminant mode means the mode signal which isan output of the discriminating circuit 6 and described above. Thesymbol "X" in FIG. 41 means that the state is not affected irrespectiveof the detection of a scene change or the discriminant mode. As shown inFIG. 41, the first switch 62 determines a selection block, and outputsthe selection block as an encoded block.

The encoded block which has been selected and output by the first switch62 is subjected to two-dimensional orthogonal transform by the DCTcircuit 8. The orthogonal-transformed data is subjected to the weightingand threshold processes or the like in the quantizing circuit 9 to bequantized to a predetermined bit number in the respective sequence. Thedata quantized by the quantizing circuit 9 are converted by the firstencoder 10 into codes suitable for the transmission path 11 and thenoutput to the transmission path 11. The data quantized by the quantizingcircuit 9 are input also to the local decoding loop 20 so thatmotion-compensation prediction is performed. The data input to the localdecoding loop 20 are subjected to inverse quantizing and inverseweighting processes in inverse quantizing circuit 12, and then subjectedto the inverse orthogonal transform by the inverse DCT circuit 13. Adecoded block which is an output of inverse DCT circuit 13 is added inthe unit of pixel to the prediction block in the adder 14 to become areproduced image. The prediction block used in this process is identicalwith that used in the subtracter 3. The block which has become areproduced image in the adder 14 is written in a predetermined addressof the video memory 15.

The memory size of the video memory 15 depends on the type of theemployed predictive method. In this embodiment, it is assumed that thevideo memory 15 consists of a plurality of field memories, and thatoutput blocks reconstructed by the local decoding loop 20 are stored ina predetermined address. These stored images are used as data of thesearch range for motion-compensation prediction. A block which issegmented from an image reconstructed from past output blocks and is ina motion estimation search range is output from the video memory 15 tothe MC circuit 16. The size of the block of the motion estimation searchrange is i pixels!×j lines! (where i≧m, j≧n, and i and j are positiveintegers). Data in the search range for motion-compensation predictionfrom video memory 15 and an input block from video memory 60 are inputto the MC circuit 16 as reference data, thereby motion vectors beingextracted.

The motion vectors extracted by the MC circuit 16 are input to the MIXcircuit 63, and combined therein with the mode signal determined by thediscriminating means 6 and the SC detection signal from the SC detectioncircuit 61. The combined signals are converted by the second encoder 18into codes suitable for the transmission path 11, and then outputtogether with the corresponding encoded block to the transmission path11. The MC circuit 16 outputs blocked signals which are segmented fromthe search range in the size (m pixels!×n lines! equal to that of theinput block, as a prediction block. The prediction block output from theMC circuit 16 is produced from past video information. The predictionblock is supplied to the second switch 64, and output in accordance withthe currently processed field, the mode signal of the decoded block andthe SC detection signal from the SC detection circuit 61. Namely, theprediction block is output from one of the output terminals of thesecond switch 64 to the subtracter 3 in accordance with the processedfield and the SC detection signal, and from the other output terminal inaccordance with the mode signal of the current decoded block, the SCdetection signal and the processed field.

The motion-compensation prediction process is shown in FIG. 42. In FIG.42, it is assumed that a scene change occurs between second field F2 andthird field F3. Since there is no scene change between first field F1and second field F2, second field F2 is predicted from first field F1.The scene change between second and third fields F2 and F3 is detected,and third field F3 becomes an intra-field in the same manner as firstfield F1. Then, fourth field F4 is predicted from third field F3. Theprediction is never made on the basis of an image which exists beforethat scene change. After the motion-compensation prediction process forfourth field F4 is completed, motion-compensation prediction is againperformed while using the next field as an intra-field. Therefore, anintra-field surely appears every four fields once themotion-compensation prediction process starts, and, when a scene changeoccurs, an intra-field exists also in the motion-compensation process.

The operation in Embodiment 10 will be summarized with reference toflowcharts of FIGS. 43 and 44. FIG. 43 is a flowchart showing the wholeoperation in Embodiment 10, and FIG. 44 is a flowchart of theprediction-field process in step S406 in FIG. 43.

At first, the field number indicative of a field in amotion-compensation prediction process unit is set to be 0 (step S401).This setting of the field number is the same as that in Embodiment 3.Since the motion-compensation process has been just started, the fieldto be initially processed is surely the first field in themotion-compensation prediction process unit and an intra-field, andtherefore the field number fn is set in step S401 to be 0. A scenechange detection flag Cfn which functions as a flag for judging whethera scene change is present is set in step S401 to be 0 for theinitialization.

Then, the characteristics of an input image are compared with those of apast image by a certain parameter to detect the presence of a scenechange (step S402). For example, the presence of a scene change isdetected by comparing the variance of values of pixels in somepredetermined areas of the past image with the variance of values ofpixels in some predetermined areas of the currently processed image.When a scene change is detected, the scene change detection flag Cfn isset to be 1 (Cfn=1), and, when a scene change is not detected, the scenechange detection flag Cfn is set to be 0 (Cfn=0).

Then, the field number fn is checked to judge whether it is 0 or thefield is the first field in the motion-compensation prediction processunit and an intra-field (step S403). If the field number fn is 0 (fn=0),this field is processed as an intra-field (step S405). In contrast, ifthe field number fn is not 0 (fn≠0), the process proceeds to next stepS404. It is judged whether the scene change detection flag Cfn is0(Cfn=0) or there is a scene change between the processed field and thereference picture required for encoding the field withmotion-compensation prediction (step S404). If the flag Cfn is 0(Cfn=0), there is no scene change, and the field to be processed isprocessed as a prediction-field (step S406). If the flag Cfn is 1(Cfn=1), there exists a scene change, and therefore the field to beprocessed is processed as an infrafield (step S405). Therefore, even inthe case that a field is in the motion-compensation prediction processunit and the field number is not 0, when a scene change is detected andthe flag Cfn is 1, the field is processed as an intra-field.

After each field is processed, the field number fn is incremented so asto indicate the next field (step S407). In an actual hardware, such afield number can be controlled by a signal from a microcomputer or thelike.

It is then judged whether the field number fn indicating the next fieldis a number indicating a field within the motion-compensation predictionprocess unit (step S408). If the field number fn is a number notindicating a field within the motion-compensation prediction processunit, for example, in FIG. 4 (if fn=4, FIG. 4), it indicates that aseries of motion-compensation prediction process units have beencompleted because, the motion-compensation prediction process unit hasbeen completed within four fields and the field number fn of anintra-field has been set to be 0. If fn<4, it is judged that the nextfield is within the motion-compensation prediction process unit, and theprocess is restarted from the scene change detection for the next fieldprocess. When the motion-compensation prediction process unit has beencompleted, it is judged whether all of the required fields have beenprocessed (step S409). This judgment can be done by, for example,checking the operation of an end switch of the high-efficiency encoder.If the next field is to be processed, in order to encode the nextmotion-compensation prediction process unit, the variables areinitialized, and the process is restarted from the scene changedetection. If the operation of the high-efficiency encoder is completed,is ended.

Next, the prediction-field process (step S406 in FIG. 43) in Embodiment10 will be described with reference to the flowchart of FIG. 44. Thefield which has been determined in step S404 in FIG. 43 to be processedas a prediction-field is blocked into a predetermined size of mpixels!×n lines! in the processed field (step S451). The blockssegmented into the size of m×n are subjected to motion-compensationprediction (step S452). The difference power P1 is calculated from adifference block which is the difference in the unit of pixel between apredetermined area of a past image and the block which has been justsegmented (step S453). The original AC power P2 kept in the state of theblock is calculated (step S454).

The calculated powers P1 and P2 are compared in magnitude with eachother (step S455). When the difference power P1 is smaller than theoriginal AC power P2, the difference block (the difference value of theblock subjected to motion-compensation prediction) is selected (stepS456). When the difference power P1 is greater than the original ACpower P2, the input block (the original which has been blocked and notsubjected to any further process) is selected (step S457). Each selectedblock is subjected to the orthogonal transform (step S458), and thequantized to a predetermined bit number which is set for each sequence(step S459). In an orthogonal transform such as DCT, for example, aquantization is performed in such a manner that a larger bit number isassigned to a DC and low-order sequences of AC, and a smaller bit numberis assigned to a high-order sequence of AC. The quantized data areconverted into codes suitable for transmission (step S460), and theencoded data are transmitted (step S461). It is judged by, for example,counting the number of processed blocks whether the process of one fieldhas been completed (step S462). If the process of one field has not yetbeen completed, the process for the next block is pursued. If theprocess of one field has been completed, the prediction field processends.

According to Embodiment 10, when a scene change occurs in amotion-compensation prediction step as shown in FIG. 42, the fieldimmediately after the scene change is set to be an intra-field, wherebya subjective appreciation of the image immediately after the scenechange can be improved.

Embodiment 11

In Embodiment 10, even when a scene change occurs in a step of amotion-compensation prediction process and the field immediately afterthe scene change is set to be an intra-field, the time-constraint lengthin the motion-compensation prediction processing step is fixed for fourfields. Namely, an intra-field surely appears every four fields once themotion-compensation prediction process starts, and, when a scene changeoccurs, an intra-field exists also in the motion-compensation processingstep. This is a configuration in which a prediction-field is replacedwith an intra-field.

According to Embodiment 11, when a scene change occurs as shown in FIG.45 and the field immediately after the scene change is set to be anintra-field, the intra-field is set to be the first field in themotion-compensation prediction process unit. That is, thetime-constraint length in the motion-compensation prediction step isvariable. Usually, the time-constraint length in the motion-compensationprediction step is set for four fields as shown in FIG. 45. When a scenechange occurs in the motion-compensation prediction step, the fieldimmediately after the scene change is set to be a new intra-field, andmotion-compensation prediction is performed in the unit of four fieldsbeginning with that field. When a scene change occurs in thismotion-compensation prediction step, the field immediately after thisscene change is set in a similar manner to be a new intra-field, andmotion-compensation prediction is performed in the unit of four fieldsbeginning with that field.

FIG. 46 is a flowchart showing the whole operation in Embodiment 11. InFIG. 46, portions designated by the same step numbers used in FIG. 43are identical with those in FIG. 43. The processes from step S401 tostep S406 are the same as in Embodiment 10. The intra-field process andprediction-field process in steps S405 and S406 are the same as inEmbodiment 10. With respect to a field which has been processed as anintra-field in step S405, the field number fn is set to be 0 (fn=0) inorder to switch to the motion-compensation prediction process unit inwhich the first field is that processed field (step S490). In Embodiment10, even when a field is processed as an intra-field, for example, thefield number fn is sequentially changed in the order of 0→1→2→3→0→ . . .as shown in FIG. 45. In Embodiment 11, when a field which is not thefirst field in the motion-compensation prediction process unit isprocessed as an intra-field, the field number fn of that field isforcedly set to be 0, and that field is set to be the first field of thenew motion-compensation prediction process unit. This allows thetime-constraint length in the motion-compensation prediction processunit to be variable. When scene changes occur at a frequency shorter interms of time than the time-constraint length in the motion-compensationprediction process unit which is set at that time, the time-constraintlength in the motion-compensation prediction process unit becomes aseries of short lengths. The processes from step S407 to step S409 arethe same as in Embodiment 10.

According to Embodiment 11, the image immediately after the scene changeis set to be an intra-field, thereby improving a subjective appreciationof the image. When the frequency of the occurrence of scene changes islonger than the time-constraint length in the motion-compensationprediction process unit and is low in level, the number of fields of anintra-field is smaller than that in Embodiment 3 so that the informationamount can be reduced.

Embodiment 12

In Embodiments 10 and 11, a process is performed while setting a field(or frame), in which a scene change is detected, to be an intra-field(or intra-frame). Alternatively, without setting as an intra-field (orintra-frame), the reference picture of the field (or frame) may be setto be intra-field (or intra-frame) belonging to the nextmotion-compensation prediction process unit.

Embodiment 12 will be described with reference to FIGS. 47(a) and (b)shows a usual motion-compensation prediction process which is performedby the method in FIG. 4. In this example, fields F10 and F14 function asan intra-field. The motion-compensation prediction is performed whilesetting these fields F10 and F14 to be the first field of themotion-compensation prediction process unit. Then, when a scene changeoccurs between field F11 and field F12 as shown in FIG. 47(b) and thescene change is detected in field F12, fields from field F12 to the lastfield (in this example, field F13) of the motion-compensation predictionprocess unit including field F12 are combined with the nextmotion-compensation prediction process unit, and fields F12 and F13 aresubjected to motion-compensation prediction in which an intra-fieldbelonging to the next motion-compensation prediction process unit isused as the reference picture. In the next motion-compensationprediction process unit combined with those fields, a normalmotion-compensation prediction and the motion compensation predictionfor the combined field as above are performed.

Embodiment 13

In Embodiment 12, the motion-compensation prediction process unit issometimes longer than the usual one. According to Embodiment 13, Pfields (or P frames) which begin with a field (or frame) wherein a scenechange is detected and have the total length corresponding to the lengthof a usual motion-compensation prediction process unit are subjected tomotion-compensation prediction.

Embodiment 13 will be described with reference to FIG. 48. FIG. 48(a)shows a usual motion-compensation prediction process which is performedby the method in FIG. 4. In this example, fields F10 and F14 function asan intra-field. While setting these fields F10 and F14 to be the firstfield of a motion-compensation prediction process unit,motion-compensation prediction is performed. Then, when a scene changeoccurs between field F11 and field F12 as shown in FIG. 48(b) and thescene change is detected in field F12, a series of four fields (this isbecause a motion-compensation prediction process unit consists of fourfields) which begins with field F12 wherein the scene change is detectedare formed into a motion-compensation prediction process unit. Field F14which, in a usual case, may be the first field of the nextmotion-compensation prediction process unit and an intra-field is set tobe an intra-field in the current motion-compensation prediction processunit, and then motion-compensation prediction is performed. Whenmotion-compensation prediction for four fields beginning with field F12,or that for fields F12 to F15 is completed, the usualmotion-compensation prediction restarts at field F16 as it was before.

Embodiment 14

In Embodiments 3 to 13, a switching is explained with reference to ascene change. A reference picture may be switched in accordance with thenumber of intra modes which have been forcedly generated in blocks.Accordingly even in a picture wherein many forcible intra modes appear,or in a case that an object which has not existed in one field beforeappears suddenly in the current field or an object which has existed inone field before disappears suddenly from the current field, a referencepicture can be switched by a similar method.

Embodiment 15

In Embodiments 3 to 13, for example, a process in whichmotion-compensation prediction as shown in FIG. 4 is performed isswitched to a process in which motion-compensation prediction as shownin FIG. 28 is performed. Before the switching, motion-compensationprediction of any kind may be performed as shown in FIG. 49. After thedetection of a scene change or the like, the process is switched tomotion-compensation prediction in which the generated information amountis reduced to a level lower than that before the switching as shown inFIG. 28.

Embodiment 16

In Embodiments 3 to 15, the motion-compensation prediction process isperformed in the unit of four fields. The number of fields which may beused as the unit is not necessarily four. The process may be performedin the unit of an arbitrary number of fields on which themotion-compensation prediction process can be performed.

According to Embodiments 3 to 16, without largely increasing the memoryamount by additionally providing a hardware as described above, evenwhen a scene change occurs in the motion-compensation prediction processunit, a reference picture is switched from the originally set referencepicture so as to minimize the influence caused by the scene change, etc.The image immediately after the scene change is set to be a referencepicture for motion-compensation prediction, and, after the detection ofthe scene change, fields before the scene change are not used as thereference picture for motion-compensation prediction. Thus transmissioncan be performed while suppressing the increase in code amount due tothe scene change to a minimum and without deteriorating the imagequality.

When motion-compensation prediction is performed before and after ascene change, the information amount of the predicted picture is usuallyincreased. Therefore, by processing the field as an intra picture withan information amount equal to that information amount, a subjectiveappreciation of the picture can be improved. While setting a pictureimmediately after a scene change as an intra picture by detecting thescene change, encoding between fields or frames can be performed so thata subjective appreciation of the picture immediately after the scenechange can be improved. When a scene change occurs, the pictureimmediately after the scene change is handled as an intra picture, andmotion-compensation prediction is performed with the intra picture beingthe first picture, whereby the number of generated intra pictures can bereduced and the amount of generated information can be reduced.

Embodiment 17

FIG. 50 is a block diagram showing the configuration of a videoinformation recording/reproducing apparatus according to the invention.In FIG. 50, the reference numerals 101 to 109 are the same as those usedin the conventional apparatus in FIG. 17 or 23.

The recording operation will be described. A video signal input to theA/D converter 101 is converted into a digital video signal, and outputto the high-efficiency encoder 110. The high-efficiency encoder 110performs the reduction of redundancy using auto-correlation of the videoinformation, the human visual characteristics and the bias of the datageneration frequency, to compress the information (its detail will bedescribed later). The output of the high-efficiency encoder 110 issupplied to the error-correction encoder 102 in which error-correctingcodes for correcting transmission errors are added to it. In thisprocess, codes having a high error correction capability and a smallinformation amount to be added are used, in order to perform ahigh-density recording and because even a small error in compressedinformation exerts an influence over a wide range. The data to whicherror-correcting codes have been added are modulated by the modulator103 to a signal suitable for magnetic heads 106 and the magnetic tape105. The modulator 103 also performs other operations such assuppression of DC and low-frequency components for the azimuthrecording, and the addition of a tracking signal which assists the tracefunction of the magnetic heads 106. The record signal which has beenmodulated by the modulator 103 is recorded through the magnetic heads106 on the magnetic tape 105. The magnetic heads 106 are mounted on therotary head drum 104 so as to be rotated by the rotation of the drum104. The so-called helical scanning recording is conducted on themagnetic tape 105.

Next, the reproduction operation will be described. The signal which hasbeen recorded by the helical scanning on the magnetic tape 105 is pickedup by the magnetic heads 106 mounted on the rotary head drum 104, andthen demodulated by the demodulator 107. The demodulated signal issubjected to the error detection and error correction by theerror-correction decoder 108. The error-corrected data are expanded bythe high-efficiency decoder 111 to be changed from the compressed codesto the original digital video signal. The reconstructed digital videosignal is converted to an analog video signal by the D/A converter 109,and then output.

FIGS. 51(a) and (b) are diagrams showing an example of the tape formatin Embodiment 17. Video information of four fields({720+360×2}×480×4/2=11.06 Mbits) is compression-encoded to about 1.3Mbits and then recorded together with an audio signal and errorcorrection codes in ten tracks. In the guardbandless recording using theazimuth method, the area recording density is 2.5 μm² /bit.

FIG. 52 is a block diagram showing the internal configuration of thehigh-efficiency encoder 110 in FIG. 50. In FIG. 52, 301 designates asubtracter which outputs the difference between an input original signaland a prediction signal, 302 designates a first switch which selectseither of the input original signal and the output of the subtracter301, 303 designates a DCT circuit which performs the orthogonaltransform of DCT, 304 designates a quantizing circuit which quantizesdata to be encoded, and 305 designates a variable-length encoder whichassigns a short code to a data of a high frequency so as to eliminatethe statistical redundancy of data. The elements 306 to 311 constitute alocal decoder for obtaining a prediction signal. The reference numeral306 designates an inverse quantizing circuit which restores thequantized data, 307 designates an inverse DCT circuit which performs theinverse DCT, 308 designates an adder which adds the prediction signal tothe difference signal to reconstruct the original signal, 309 designatesa video memory which stores local-reconstructed video data, 310designates a motion-compensation circuit which detects motion from theinput original signal and outputs the next prediction data, and 311designates a second switch which switches data to be input to the adder308.

The operation of the high-efficiency encoder 110 will be described. Theinitial field of a recording unit block is encoded as an intra-fieldwhich does not use the interplane prediction. Since the first switch 302selects the upper contact, an input digital video signal is subjected toorthogonal transform by the DCT circuit 303. The transformed data arequantized by the quantizing circuit 304, and encoded by thevariable-length encoder 305 to a variable-length code such as a Huffmancode, to be output. At the same time, the quantized data areinverse-quantized by the inverse quantizing circuit 306, and thensupplied to the inverse DCT circuit 307. In the inverse DCT circuit 307,the orthogonal-transformed data are inverted to the original video dataand then output to the adder 308. In the intra-field, the second switch311 also selects the upper contact so that the one input of the adder308 is zero. Therefore, the output of the inverse DCT circuit 307 issupplied as it is to the video memory 309 to be stored thereinto.

In encoding of the next field, interplane prediction is used. Ininterplane prediction, both the first and second switches 302 and 311select their lower terminal. An input digital video signal enters thesubtracter 301 and motion-compensation circuit 310. Themotion-compensation circuit 310 compares the stored picture with theinput picture, and outputs motion vectors of the input picture and aprediction picture to be used in the predictive encoding. The subtracter301 calculates the difference between the input picture and theprediction picture, and output it as a prediction difference signal tothe DCT circuit 303. Compared with a raw video signal, a predictiondifference signal has a smaller information amount as the predictionaccuracy becomes higher. For example, a prediction difference signal foran entirely still picture is zero. In the same manner as the initialfield, the data input to the DCT circuit 303 are subjected by the DCTcircuit 303 and quantizing circuit 304 to the orthogonal transform andquantization, and then converted into variable-length codes by thevariable-length encoder 305 to be output. On the other hand, thequantized data are supplied through the inverse quantizing circuit 306to the inverse DCT circuit 307 to be subjected to the inversequantization and inverse orthogonal transform, and then sent to theadder 308. Since the prediction picture used in the process of obtainingthe prediction difference is supplied to the other input terminal ofadder 308, the output of the adder 308 is the same as the input picture.The output of the adder 308 is stored into the video memory 309. In thesame manner as described above, the process of encoding n fields ispursued.

FIG. 53 shows an example of the variation of the amount of generatedcodes for each frame. In this example, it will be noted that anintra-field which does not use the interplane prediction is placed afterevery seven fields, thereby increasing the information amount. FIG. 54shows an example of the relationship between recorded information ineach field and the writing in tracks. In this example, data of fourfields are recorded in ten tracks. The data amount of one field may notbe an integral multiple of the recording capacity of a track.

Embodiment 18

In Embodiment 17, data of four fields are recorded in ten tracks. It isnot necessary to always record data in ten tracks, and data may berecorded in eight or six tracks depending on the amount of informationto be recorded.

As described above, in Embodiments 17 and 18, since signals of aplurality of fields or frames are collected in one recording unit to berecorded in a predetermined number of tracks, all of the recorded fieldscan be reconstructed by performing reproduction processing on apredetermined number of tracks. Therefore, the embodiments can cope withthe special reproduction and edition required in a VTR. Since the numberof tracks to be used in the recording is selected depending on theamount of information to be recorded, there is no wasted track, therebyrecording and reproduction can be performed for a long period.Further-more, since it is not necessary to control information to berecorded so as to match the recording capacity of each track, there isno useless portion in each track, with the result being that recordingcan be performed efficiently. Moreover, since an intra-picture whichdoes not use interplane prediction surely exists in each recording unit,a reconstructed picture can be easily obtained even in a specialreproduction such as a speed search, and the amount of information to berecorded can be reduced as compared with a prediction picture which usesthe interplane prediction.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within metesand bounds of the claims, or equivalence of such metes and boundsthereof are therefore intended to be embraced by the claims.

What is claimed is:
 1. A high efficiency encoder comprising:means forsegmenting a digitized video signal into blocks of a predetermined size,and for encoding the blocks; prediction means for creating anintra-field or an intra-frame in which motion-compensation prediction isnot performed, for every n fields or frames (n is an arbitrary integernot less than 2), and for performing motion-compensation prediction withan intra-field or an intra-frame on each block for the succeeding (n-1)fields or (n-1) frames; means for comparing a first information amountof each block with a second information amount of each block, the firstinformation amount being obtained when motion-compensation prediction isperformed and the second information amount being obtained whenmotion-compensation prediction is not performed; and selection meansfor, when said second information amount is smaller than said firstinformation amount, selecting a forced intra mode as a coding mode.
 2. Ahigh-efficiency encoder comprising:encoding means for segmenting adigitized video signal into blocks of a predetermined size, and forencoding the blocks; prediction means for creating an intra-field or anintra-frame in which motion-compensation prediction is not performed,for every n fields or frames (n is an arbitrary integer not less than2), and for performing motion-compensation prediction with anintra-field or an intra-frame on each block for the succeeding (n-1)fields or (n-1) frames; and means for comparing a first informationamount of each block with a second information amount of each block, thefirst information amount being obtained when motion-compensationprediction is performed and the second information amount being obtainedwhen motion-compensation prediction is not performed, wherein, for afield or frame where the number of blocks in which the secondinformation amount of the block is smaller than the first informationamount of the block is greater, said prediction means does not performmotion-compensation prediction, and said encoding means performsencoding as a forced intra-field or a forced intra-frame.
 3. Ahigh-efficiency encoder comprising:encoding means for segmenting adigitized video signal into blocks of a predetermined size, and forencoding the blocks; prediction means for creating an intra-field or anintra-frame in which motion-compensation prediction is not performed,for every n fields or frames (n is an arbitrary integer not less than2), and for performing motion-compensation prediction with anintra-field or an intra-frame on each block for the succeeding (n-1)fields or (n-1) frames; and means for comparing a first informationamount of each block with a second information amount of each block, thefirst information amount being obtained when motion-compensationprediction is performed and the second information amount being obtainedwhen motion-compensation prediction is not performed, wherein, for afield or frame where the number of blocks in which the secondinformation amount of the block is smaller than the first informationamount of the block is greater, said prediction means does not performmotion-compensation prediction, and said encoding means performsencoding as a forced intra-field or a forced intra-frame, and, for atleast (n-1) fields or (n-1) frames following said forced intra mode,said prediction means again performed motion-compensation predictionwith a forced intra-field or a forced intra-frame.
 4. A high efficiencyencoder, comprising:a blocking circuit for segmenting a digitized videosignal into blocks of predetermined size; a block processing circuitencoding blocks of a selected field or frame; a selecting circuitselects, every n fields or frames, a field or frame output by theblocking circuit as an intra-field or intra-frame, the intra-field orintra-frame being the selected field or frame, and selects one of theinput blocks and predicting blocks of the successive (n-1) fields or(n-1) frames as the selected field or frame; a motion-compensationpredicting circuit generating the predicting blocks of the successive(n-1) fields or (n-1) frames based on the intra-field or intra-frame,wherein the selecting circuit includes:a determiner which determineswhether a sum of the absolute values of the blocks of a field or frameoutput by the blocking circuit is smaller than a sum of the absolutevalues of the predicting blocks of one of the successive (n-1) fields or(n-1) frames; and a switch which selects one of the successive (n-1)fields or (n-1) frames output by the blocking circuit and the predictingblocks of the successive (n-1) fields or (n-1) frames as the selectedfield or frame based on output from the determiner.
 5. A high efficiencyencoder, comprising:a blocking circuit for segmenting a digitized videosignal into input blocks of predetermined size; first memory storing theinput blocks; a block processing circuit encoding blocks of a selectedfield or frame; a motion-compensation predicting circuit predictingblocks of successive fields or frames from a reference picture; aselecting circuit outputting one of a predicting block and an inputblock based on information content of the predicting block and the inputblock; a second memory storing blocks output by the selecting circuit;and wherein said selecting circuit selects as the selected field orframe, blocks stored in one of the first memory and the second memory,based on one of a number of predicting blocks output to the secondmemory and a number of input blocks output to the second memory.
 6. Theencoder of claim 5, further comprising:a reference picture supplycircuit which supplies, every n fields or frames, a field or frameoutput by the blocking circuit as the reference picture, and supplies asthe reference picture for the nth field or frame, an n-2 selected fieldor frame.
 7. The encoder of claim 5, further comprising:a referencepicture supply circuit which supplies, every n fields or frames, a fieldor frame output by the blocking circuit as the reference picture.
 8. Theencoder of claim 5, further comprising:a reference picture supplycircuit which supplies, every n fields or frames, a field or frameoutput by the blocking circuit, and supplies as the reference picturefor the nth field or frame, an n-1 selected field or frame.
 9. Theencoder of claim 5, wherein the block processing circuit includes atwo-dimensional DCT circuit which orthogonally transforms the selectedfield or frame.
 10. The encoder of claim 5, further comprising areference picture supply circuit which supplies as the referencepicture, the input blocks stored in the first memory when the selectingmeans selects the input blocks stored in the first memory as theselected field or frame.